drm/i915: unload: fix unpin_work related races

Kill any outstanding unpin_work when destroying the corresponding
crtc. Then flush the workqueue before the gem teardown, in case
any unpin work is still outstanding.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>