anv: Stop using the i915 engines API for now

We're currently in the process of trying to clean up some of the i915 uAPI. One of those messes is the engine/context/vm APIs. The current Mesa code uses that API completely differently from media and compute. It was landed a little over 6 weeks ago, doesn't enable any new features today, and isn't in any release. As far as kernel uAPI backwards- compatibility goes, it should be safe to shut it off and pretend it never happened. This gives us a bit more flexibility as we re-evaluate the kernel APIs.