userspace mappings before doing other flushes. Only changes
  anything when userspace does racy stuff against itself.

* also flush GTT domain. This is a noop, but still try to keep the
  bookkeeping correct.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Eric Anholt <eric@anholt.net>
¹_