drm/nvc0/fifo: ignore bits in PFIFO_INTR that aren't set in PFIFO_INTR_EN

PFIFO_INTR = 0x40000000 appears to be a normal case on nvc0/nve0 PFIFO,
the binary driver appears to completely ignore it in its PFIFO interrupt
handler and even masks off the bit (as we do) in PFIFO_INTR_EN at init
time.

The bits still light up in the hardware sometimes though, so lets just
ignore any bits we haven't explicitely requested.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nvc0_fifo.c

@@ -373,7 +373,8 @@ nvc0_fifo_isr_subfifo_intr(struct drm_device *dev, int unit)
 static void
 nvc0_fifo_isr(struct drm_device *dev)
 {
-	u32 stat = nv_rd32(dev, 0x002100);
+	u32 mask = nv_rd32(dev, 0x002140);
+	u32 stat = nv_rd32(dev, 0x002100) & mask;
 
 	if (stat & 0x00000100) {
 		NV_INFO(dev, "PFIFO: unknown status 0x00000100\n");

drivers/gpu/drm/nouveau/nve0_fifo.c

@@ -345,7 +345,8 @@ nve0_fifo_isr_subfifo_intr(struct drm_device *dev, int unit)
 static void
 nve0_fifo_isr(struct drm_device *dev)
 {
-	u32 stat = nv_rd32(dev, 0x002100);
+	u32 mask = nv_rd32(dev, 0x002140);
+	u32 stat = nv_rd32(dev, 0x002100) & mask;
 
 	if (stat & 0x00000100) {
 		NV_INFO(dev, "PFIFO: unknown status 0x00000100\n");