drm/nouveau/pm: introduce ram reclocking helper

This will probably result in more lines of code, however, we're going to
have at least 3 slightly different implementations of this very soon and
I'd rather keep the ram reclocking logic separate from the hw specifics.

DDR2/DDR3/GDDR3 implemented thus far, others will be added as necessary.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Martin Peres <martin.peres@labri.fr>