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

[Martin Peres]

  Hi,

Here is an updated version, all in one patch. When we agree on the code, 
I'll split it into 3 patches.

The new code is way better. As suggested on IRC, I also disable IRQs and 
PFIFO cache pull and push.

Please have a look and test it. This patch needs to get upstream as it 
gives a reliable way to change the clock even in the middle of a game.

It is not perfect yet, changing the clocks too often (like several 
thousand of times) lead to an hung card :)

Regards,

Martin