Apple absolutely has graphics driver teams for iOS and OS X. They control a significant portion of the driver stack, not to mention the rendering APIs.
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've spoken to the driver teams for both Imagination and iOS. For anything regarding iOS, the Imagination team will always say "We can't answer that, speak to the iOS team." For anything that's not in the public docs, the iOS team will respond "We can't answer that." ;)
