drm/nv50/pm: rewrite clock management, and switch to the new pm hooks

This area is horrifically complicated on these chipsets, and it's likely we
will need at least a few more tweaks yet.

Oh yes, and it's completely disabled on IGPs for the moment.  From traces,
things look potentially different there yet again.  Sigh...

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