drm/nouveau/disp: expose head event class

Also fixes vblank interrupts being left enabled when they're not meant
to be as a result of races/bugs in previous event handling code.

v2:
- use ?: (lyude)

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>