nouveau: reinstate fencing on screen destroy

nouveau: reinstate fencing on screen destroy

As it turns out, the wait is required as the driver expects for
rendering to be quiesced on exit. This can trigger channel failures,
which in turn trigger recovery. This can fail and destroy the whole
system.

Fixes: 28a781323fb ("nouveau: change fence destruction logic on screen destroy")
References: https://gitlab.freedesktop.org/mesa/mesa/-/issues/4223
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8867>