drm/nv40/pm: write nv40-specific reclocking routines

Not 100% perfect yet, but a good start towards what it'll look like in the
end.

Actually seems stable on a NV44 I have here, as much as running around OA
for a fair amount of time constantly switching between performance levels
can prove..

My NV49 isn't quite so happy, and semaphores mess up somehow (sometimes) as
a result of the memory reclocking.

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