drm/nouveau/fifo/nv04: remove the loop from the interrupt handler

Complete bong hit (and not the last...), the hardware will reassert the
interrupt to PMC if it's necessary.

Also potentially harmful in the face of interrupts such as the non-stall
interrupt, which remain active in NV_PFIFO_INTR even when we don't care
about servicing it.

It appears (hopefully, fdo#87244), that under certain loads, the methods
may pass quickly enough to hit the "100 spins and kill PFIFO" thing that
we had going on.  Not ideal ;)

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>