drm/i915: prevent possible pin leak on error path

We should not hit this under any sane conditions, but still, this does not
looks right.

CC: Chris Wilson <chris@chris-wilson.co.uk>
CC: Daniel Vetter <daniel.vetter@ffwll.ch>
CC: stable@vger.kernel.org
Reported-by: Herton Ronaldo Krzesinski <herton.krzesinski@canonical.com>
Reviewed-by: Chris Wlison <chris@chris-wilson.co.uk>
Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>