drm/nouveau: Use unlocked gem unreferencing

For drm_gem_object_unreference callers are required to hold
dev->struct_mutex, which these paths don't. Enforcing this requirement
has become a bit more strict with

commit ef4c6270
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Oct 15 09:36:25 2015 +0200

    drm/gem: Check locking in drm_gem_object_unreference

Cc: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459330852-27668-2-git-send-email-daniel.vetter@ffwll.ch

drivers/gpu/drm/nouveau/nouveau_display.c

@@ -296,7 +296,7 @@ nouveau_user_framebuffer_create(struct drm_device *dev,
 err:
 	kfree(nouveau_fb);
 err_unref:
-	drm_gem_object_unreference(gem);
+	drm_gem_object_unreference_unlocked(gem);
 	return ERR_PTR(ret);
 }