drm/i915: refleak along pin() error path.

A missing unreference if the user calls pin() a second time on a pinned
buffer.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@linux.ie>

drivers/gpu/drm/i915/i915_gem.c

@@ -2755,6 +2755,7 @@ i915_gem_pin_ioctl(struct drm_device *dev, void *data,
 	if (obj_priv->pin_filp != NULL && obj_priv->pin_filp != file_priv) {
 		DRM_ERROR("Already pinned in i915_gem_pin_ioctl(): %d\n",
 			  args->handle);
+		drm_gem_object_unreference(obj);
 		mutex_unlock(&dev->struct_mutex);
 		return -EINVAL;
 	}