[Nouveau] MEMX improvements + DDR 2/3 MR generation

[Roy Spliet]

Patch 1 and 2 implement wait-for-vblank, required to remove flicker when reclocking memory
Patch 3 and 4 allow me to do things between waiting for VBLANK and disabling FB, like pause PFIFO and wait for the engines to idle. This minimises the time PFIFO is paused, thus maximises performance.
The rest of the patches speak for themselves. As the actual memory reclocking script is still somewhat prone to changes, this will follow later. Please review and merge if positive!
Thanks,

Roy