I interviewed with apple and many of the people I interviewed with were explicitly on their mobile driver team. We discussed hardware and driver stack details. They made it clear that yes, they are partially responsible for the driver stack.
I also have explicitly had my statement confirmed by game & driver developers for desktop OS X; I can't say with 100% certainty that Apple owns the iOS stack, but I am relatively certain that it is true given that it is 100% known to be true on OS X.
Common-sense wise, I find it highly unlikely that iOS would be bolted directly to the powerVR graphics stack. It would make it too hard for them to move to another graphics vendor. So I think it is almost a certainty that they own some, if not all of the graphics stack, even if it is derived from proprietary PowerVR code & hardware.
I also have explicitly had my statement confirmed by game & driver developers for desktop OS X; I can't say with 100% certainty that Apple owns the iOS stack, but I am relatively certain that it is true given that it is 100% known to be true on OS X.
Common-sense wise, I find it highly unlikely that iOS would be bolted directly to the powerVR graphics stack. It would make it too hard for them to move to another graphics vendor. So I think it is almost a certainty that they own some, if not all of the graphics stack, even if it is derived from proprietary PowerVR code & hardware.