drm/nouveau/pm/gf100: only use PBFB_BROADCAST.PM_UNK100 for PBFB signals

High level hardware events related to PBFB will monitor all partitions.
While we are at it, fix bitfield for this mux.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>