[Nouveau] nv50: wfb patches

[Maarten Maathuis]

XSHM could probably be fixed, but that would have to be done on a
larger scope (as in for everything, not just exa). This could
negatively impact those with no acceleration at all.

About the untiling + copy approach, this is what classic exa does, and
you are practically forced to keep a software copy around always.
Otherwise you get excessive copies. I know right now that reading
memory on uncached memory isn't so great, but maybe in the future we
could use the gart also.

wfb is just a "simple" way of handling things, exa's migration
scheme's aren't easy.