drm/nouveau/gsp: fix rm shutdown wait condition

Though the initial upstreamed GSP-RM version in nouveau was 535.113.01,
the code was developed against earlier versions.

535.42.02 modified the mailbox value used by GSP-RM to signal shutdown
has completed, which was missed at the time.

I'm not aware of any issues caused by this, but noticed the bug while
working on GB20x support.

Signed-off-by: Ben Skeggs <bskeggs@nvidia.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Timur Tabi <ttabi@nvidia.com>
Tested-by: Timur Tabi <ttabi@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>