i915: add missing return in error path.

Signed-off-by: Dave Airlie <[e-mail]>

drivers/gpu/drm/i915/i915_gem.c

···
 	if (ret != 0) {
 		drm_gem_object_unreference(obj);
 		mutex_unlock(&dev->struct_mutex);
+		return ret;
 	}
 
 	offset = args->offset;