[Intel-gfx] Tracking VMA

[Chris Wilson]

One issue with the current VMA api is that callers do not take ownership
of the VMA they pin for their use, and corresponding never explicitly
unpin it. Being able to track the VMA they are using, imo, allows for
simpler code that is more easily verified (and is faster and more
accurate - less guessing over state).

However, at the start are patches to take advantage of lockless request
lookups.
-Chris