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>

drivers/gpu/drm/i915/intel_display.c

@@ -6317,7 +6317,7 @@ static int intel_gen7_queue_flip(struct drm_device *dev,
 	default:
 		WARN_ONCE(1, "unknown plane in flip command\n");
 		ret = -ENODEV;
-		goto err;
+		goto err_unpin;
 	}
 
 	ret = intel_ring_begin(ring, 4);