drm/i915: Attempt to uncouple object after catastrophic failure in unbind

If we fail to flush outstanding GPU writes but return the memory to the
system, we risk corrupting memory should the GPU recovery and complete
those writes. On the other hand, if we bail early and free the object
then we have a definite use-after-free and real memory corruption.
Choose the lesser of two evils, since in order to recover from the hung
GPU we need to completely reset it, those pending writes should
never happen.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>