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>