drm/nouveau: wait on fence after bo move if validating for another channel

Not an ideal solution, but it'll do for the moment for correctness.  We
need to come up with a nicer way to manage inter-channel sync, the hw
is unfortunately a little lacking in this area.

Should fix some resume corruption, as well as corruption that may be seen
while under memory pressure.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>