drm/nouveau: fix nv84 fence context leak

uevent based fences hold a reference to the fence context,
just like the legacy ones. So they need to drop this reference
in the same way.
Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_fence.c

@@ -586,5 +586,5 @@ static const struct dma_fence_ops nouveau_fence_ops_uevent = {
 	.enable_signaling = nouveau_fence_enable_signaling,
 	.signaled = nouveau_fence_is_signaled,
 	.wait = dma_fence_default_wait,
-	.release = NULL
+	.release = nouveau_fence_release
 };