Like other cases, it should use rcu protected 'chan' rather
than 'fence->channel' in nouveau_fence_wait_uevent_handler.

Fixes: 0ec5f02f ("drm/nouveau: prevent stale fence->channel pointers, and protect with rcu")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>