[Intel-gfx] vmap consolidation

[Chris Wilson]

We have a couple of pieces of code that wish to take further advantange
of prolonged vmappings: execlists (ringbuffers), the cmdparser and the GuC
(workqueues). This series refactors the whole-object vmapping code and
caches it on the drm_i915_gem_object until it is released along with the
object's pages (i.e. under memory pressure or at the end of the object's
lifetime). This was stalled until we had a notifier in place from the
mm/vmalloc for when we ran out of vmalloc arena (quite possible on 32bit
machines which only reserve ~128MiB globally).

This should unblock a couple of perf.orientated series, but doesn't offer
any new features for itself.
-Chris