drm/i915: Unpin the flip target if we fail to queue the flip

commit 83d4092b0381e5dd6f312b2ec57121dcf0fcbade upstream.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[bwh: Backported to 3.2:
 - adjust context
 - we don't have intel_unpin_fb_obj(); use i915_gem_object_unpin()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>