drm/nouveau/pwr: add support for GK20A

This patch adds PWR support for GK20A. But instead of adding the PWR
features like firmware loading and communication with PMU firmware, we
add the DVFS (Dynamic Voltage and Frequency Scaling), which is one of
the PMU firmware's jobs on dGPUs, in this patch. This refers to the
idle signals provided by the NVIDIA hardware and tries to adjust the
performance level based on the calculated target. The reclocking policy
can be fine-tuned later when we have more real use cases.

Signed-off-by: Vince Hsu <vinceh@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>