drm/nouveau/privring: ack interrupts the same way as RM

[ Upstream commit e05e06cd34f5311f677294a08b609acfbc315236 ]

Whatever it is that we were doing before doesn't work on Ampere.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>