drm/nouveau/fifo/g84-: ack non-stall interrupt before handling it

Closes a very unlikely race that can occur if another NonStallInterrupt
method passes between checking fences and acking the previous interrupt.

With this change, the interrupt will re-fire under such conditions.

Tested-by: Tobias Klausmann <tobias.johannes.klausmann@mni.thm.de>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

diff --cc drivers/gpu/drm/nouveau/core/engine/fifo/nv04.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/engine/fifo/nvc0.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/engine/fifo/nve0.c
Simple merge