[Nouveau] Add pause/unpause methods for PFIFO & PGRAPH. Use them to get stable clock changes

[Martin Peres]

  Hi,

With these patches, I get reliable clock changes on my nv86. It should 
work from nv50 to nvc0 (non-included).

Now, we need to wait for vblank. This is on my TODO but if someone knows 
how to do so, please send me pointers to it or implement it yourself :)

Regards,

Martin