DRI2 fixes for 1.8-branch

[Peter Hutterer]

I've got a few DRI2 cherry-picks for 1.8 on 
git://people.freedesktop.org/~whot/xserver.git server-1.8-branch

Cherry-picking them was more complicated than I hoped for since we had one
of the patches reverted, then others on top, etc. And I don't know what I'm
doing when it comes to DRI2.

So, please have a look at these and check if they're correct. If they are,
I'll merge them into 1.8, if not I'll just leave them.

Cheers,
  Peter