[Intel-gfx] First class i915_vma

[Chris Wilson]

For a long time, i915_vma has been the way we actually interact with the
hardware, as opposed to simply using the object as long ago. Accepting
this and using the i915_vma as the primary object for tracking such
interactions leads to a great deal of simplification and allows us to
move more hardware tracking from the object to the vma, fixing a number
of long outstanding bugs.

It just happens that the actual conversion got tied up in the middle of
refreshing the GPU error state...
-Chris