drm/nouveau/mc: move NV_PMC_ENABLE bashing to chipset-specific code

Ampere needs different handling here, most of what we touch has moved.

We probably want to refactor these interfaces in general, but I'm not
yet sure how they should look, this will get the job done for now.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>