We already have a function that does what this code was doing, so let's
use that instead.

We don't need to keep calling the same function over and over again in a
loop, especially when the behavior is slightly non-trivial. We can just
keep a reference to the looked up location and do all the checking and
assignments based off it instead.

It removes a mapping previously created with the MapBufferEx ioctl.

Also removed some dead code and added memory map consistency asserts.

Also moved the GPU MemoryManager class to video_core since it makes more sense for it to be there.