drm/nouveau/kms: pass event mask to hpd handler

Will be moving the DP link status check / re-train here so it's safe
from racing with modeset routing changes.

MST message handling etc. will remain where it is.

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