drm/gk104/pwr: implement PGOB disable method

As documented at:

ftp://download.nvidia.com/open-gpu-doc/gk104-disable-graphics-power-gating/1/gk104-disable-graphics-power-gating.txt



NVIDIA were not able document the steps necessary to detect whether this
is required or not at this time.  However, they did confirm that this
procedure is safe to perform unconditionally on GK104/6.  GK107 does not
have the power gating feature, and it was recommended that we do not
perform these steps there as the effects were not verified.

The disable path is from observing the binary driver, and not
documented in the link above.

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