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

[Martin Peres]

  Hi,

Here is an updated set of patches (in regards on comments got on IRC and 
an handling error when using several channels).

These updated patch allow me to run 5 glxgears at the same time and 
reclock the card almost 100 times per second without crashing.

Don't panic if you get theses in your logs:
PGRAPH: wait for idle fail: 00000000 00000000 00000000 00000103!
PGRAPH: PGRAPH paused while running a ctxprog, NV40_PGRAPH_CTXCTL_0310 = 
0x11

It means that we tried to reclock during a context switch. It would be 
safe to continue as when the program is at 0x11, it means it waits for 
another process (which is stopped because we stopped PFIFO before). I 
could allow to get this value (ie continue and clock), but if someone 
changes the ctxprog, he would have to update it here.

For the moment, I don't mind if from time to time, setting the clocks fail.

Waiting for your comments.

Regards,

Martin