drm/nouveau/tmr: ack interrupt before processing alarms

BugLink: http://bugs.launchpad.net/bugs/1694621

commit 3733bd8b upstream.

Fixes a race where we can miss an alarm that triggers while we're already
processing previous alarms.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

drivers/gpu/drm/nouveau/nvkm/subdev/timer/nv04.c

@@ -76,8 +76,8 @@ nv04_timer_intr(struct nvkm_timer *tmr)
 	u32 stat = nvkm_rd32(device, NV04_PTIMER_INTR_0);
 
 	if (stat & 0x00000001) {
-		nvkm_timer_alarm_trigger(tmr);
 		nvkm_wr32(device, NV04_PTIMER_INTR_0, 0x00000001);
+		nvkm_timer_alarm_trigger(tmr);
 		stat &= ~0x00000001;
 	}