drm/nouveau/fault: add explicit control over fault buffer interrupts

The GPU will continually fire interrupts while a fault buffer GET != PUT,
and to stop the spurious interrupts while the handler does its thing, we
were disabling the fault buffer temporarily.

This is not actually a great idea to begin with, and made worse by Volta
resetting GET/PUT when it's reactivated.  So, let's not do that.

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