[Intel-gfx] Multiple timelines, take 2

[Chris Wilson]

A minor hitch in handling the unification of vma + obj retirements is
that we trigger a recursive use-after-free if we try to purge an object
with an active reference. One way to side-step that issue was to pull in
the deferred free work, which pulled in dependencies on the separate
mm.lock and a few rearrangements to make that possible.
-Chris