drm/nouveau/kms/nv50: use the correct state for base channel notifier setup

Fixes: 857263 ("drm/nouveau: Handle drm_atomic_helper_swap_state failure")
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Tested-by: Lyude Paul <lyude@redhat.com>
Reviewed by: Lyude Paul <lyude@redhat.com>