You ask:
Who was the first philosopher to describe what we now call artificial intelligence?
Alan Turing is the first thinker to begin to address an artificial general intelligence. But we need to discuss what it means in contemporary terms of the use of the term "artificial intelligence".
First, AI is a modern conception because it relies on a heavily psychological characterization, the language which comes from modern psychology.. From Russell and Norvig, let us introduce some ideas that are relevant to what characterizes AI:
- the capacity to solve problems
- knowledge and the ability to reason with it
- planning and intention and action
- uncertainty and ways of dealing with it
- learning
- communication and perception
These would be the properties taken as a sum to constitute artificial general intelligence. Modern AI doesn't begin to approach it, but limited aspects of it are currently infusing the world with intelligence on a limited basis. In fact, we have industries devoted to each of the aspects above. Automated theorem provers, expert systems, decision support systems, machine learning, and robotics is flourishing right now. But each is an aspect of AGI, so if you conflate AI with autonomous mechanical beings, such a philosopher would have to have addressed all of them. No such philosopher, per se, exists.
But, if you are looking for an ancient Greek who saw the universe as a mechanical apparatus in which men could meet all of these characteristics, read your mythology, and not your philosophy. As others have noted, only the gods of Olympus were thought to have the ability to constitute mechanical systems with the properties above. As others have noted, consider Talos. But Hephaestus was a busy god, and his automata included people (theoi.com). Here's WP's article on automata which might give you other clues.
The view in the other direction, that animals were merely automatons was advocated by Descartes much later. Here's a reference request in PhilSE about what Descartes proposes. Here again, we can see a philosopher advocating for what the ancient Greeks had envisioned and what Čapek coined in his drama with the introduction of the term: robots. So, it's fair that the ancient Greeks clearly had envisioned both robots and androids with general intelligence, but they wouldn't have had the capacity to do it in the modern language of AI, because they didn't have the vocabulary and theory of modern psychology (especially a computational theory of mind (SEP)) and other sciences to do it.
Now, if you're willing to constrain your conception to the aspect of the automation of knowledge, then Llull and Leibniz are relevant. Llull invented a universal logic anticipating Leibniz characteristica universalis and the calculus ratiocinator. While Llull wanted to an irrefutable language to discuss and prove God, Leibniz, a co-inventor of calculus, had an understanding of technology of early modern science, so it is conceivable that he was envisioning a thinking system, at least according to the founder of Cybernetics, Norbert Weiner. From WP's latter article:
A contrasting point of view stems from synthetic philosophy and fields such as cybernetics, electronic engineering, and general systems theory. It is little appreciated in analytic philosophy. The synthetic view understands the calculus ratiocinator as referring to a "calculating machine". The cybernetician Norbert Wiener considered Leibniz's calculus ratiocinator a forerunner to the modern day digital computer
Of course, Charles Babbage is widely recognized as cooking up such details for the actual automation of computation a couple of hundred years later. Inspired mechanical contraptions, Babbage designed, though never successfully built such devices. But he certainly can be credited with anticipating modern computing architectures offered by Zuse, Atanasoff and Berry, and Mauchly and Eckert.
So, the point is it all depends on what you mean by AI. There seem to be a number of philosophical distinctions that one can view the question through: living and non-living, intelligent and dumb, organic and inorganic, mental and physical. It should be noted that even the definition of intelligence today is contentious. There are two camps, some that align with a tradition whose most famous representative these days is Gardner and his MI theory. But the G factor and IQ as conceived by men like Cattell and Horn is another formalization of intelligence.
So, if we envision the philosopher who is best thought of as working
on the question that embodies artificial general intelligence based on a computational theory of mind, then we are left with Alan Turing, who in his paper envisioned modern computational systems that pass for human thinkers and would have to learn like children. (The Turing Test is famous, but the paper shows he had an eye on how such systems would learn in an off-hand remark later in the paper.) This is why he is recognized as a father to both computer science (he was a logician by PhD thesis) and AI (a term coined by McCarthy at Dartmouth long after WWII). So, until you commit to a definition of artificial intelligence, the question is in a way exploratory and imprecise.