drm/nouveau: use different register to wait for secret scrubber

Fixes long delay when waiting for scrubber on some secret engines.
The exit interrupt seems to not always be generated, so use secret
scrubber active register instead.

Later fuc engines also no longer generate an interrupt, so don't wait
there.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>