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>
Signed-off-by: Julien Cristau <jcristau@debian.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>