drm/nouveau/mc: expose device enable/disable separately, as well as reset

There are cases where subdevs need to perform additonal actions around
the master reset, so we want to expost the operations separately.

This commit also adds a flag to the NV_PMC_ENABLE bitfield definitions
which allow skipping the automatic reset() called from core/subdev.c.

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