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>