drm/nouveau/gr/gf100: wait for GR idle after GO_IDLE bundle

After submitting a GO_IDLE bundle, one must wait for GR to effectively
be idle before submitting the next bundle. Failure to do so may result
in undefined behavior in some rare cases.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reported-by: Kary Jin <karyj@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>