drm/nouveau: idle channel before releasing notify object

Unmapping it while it's still in use (e.g. by M2MF) can lead to page faults
and a lot of TRAP_M2MF spam in dmesg.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>