drm/i915: Drop any active reference before unbinding

Before we process the final unbind on an object and move it to the
unbound list, it is semantically cleaner if there are no more active
references to the object. (An active reference would imply that it was
still being accessed by the GPU after it became inaccessible.) The
caveat is that all callsites must be prepared for the object to
disappeared during the unbind - i.e. they must hold their own reference.

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