drm/nouveau/pmu/gk104: implement a hackish workaround for a hw bug

Only a handful of machines have this enabled by default, where it's been
proven to work.  The workaround can be explicitly enabled with a module
option also.

Still waiting on feedback from NVIDIA for a proper idea of exactly what
this fix is doing, and how to implement it properly.

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