drm/nouveau/gr/gf100: Clear notify interrupt

Notify interrupt is only used for cyclestats. We can just clear it and
avoid an "unknown stat" error that gets printed to dmesg otherwise.

Signed-off-by: Lauri Peltonen <lpeltonen@nvidia.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>