drm/nvd0/disp: handle yet another interrupt

Spotted while messing with overlay channels (probably as a result of
sending a similar "disable" sequence as we do for the flip channels).

The value in 0x61008c was 0x20, which one would reasonably guess is
"bit 5 == something to report about evo channel 5" - but who knows.

Spotted the binary driver getting this too, and it appears to not do
anything exciting as a result.  So, handle it the same way and avoid
an IRQ storm.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nvd0_display.c

@@ -1585,6 +1585,12 @@ nvd0_display_intr(struct drm_device *dev)
 	struct nvd0_display *disp = nvd0_display(dev);
 	u32 intr = nv_rd32(dev, 0x610088);
 
+	if (intr & 0x00000001) {
+		u32 stat = nv_rd32(dev, 0x61008c);
+		nv_wr32(dev, 0x61008c, stat);
+		intr &= ~0x00000001;
+	}
+
 	if (intr & 0x00000002) {
 		u32 stat = nv_rd32(dev, 0x61009c);
 		int chid = ffs(stat) - 1;