[Intel-gfx] Speed up resume by focused clflushing

[Chris Wilson]

If we track the domain changes from hiberation, we can forgo the forced
flushing of objects inside i915_gem_restore_gtt_mappings() and instead
rely on lazy flushes of objects via the normal domain handling. The
caveat are objects currently in use by the hardware, which need to be
coherent upon resume for the ongoing access.
-Chris