drm/nouveau/gr/gk104-gk20a: call pmu to disable any power-gating before ctor()

On some of these chipsets, reading NV_PGRAPH_GPC_GPM_PD_PES_TPC_ID_MASK
can trigger a PRI fault and return an error code instead of a TPC mask,
unless PGOB has been disabled first.

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