drm/nouveau/pm: implement NVIF_PERFMON_V0_QUERY_DOMAIN method

This allows to query the number of available domains, including the
number of hardware counter and the number of signals per domain.

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