drm/nva3/pm: make pll->pll mode work

This probably wants a cleanup, but I'm holding off until I know for sure
how the rest of the things that need doing fit together.

Tested on NVS300 by hacking up perflvl 1 to require PLL mode, and switching
between perflvl 3 and 1.

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