I wanted to add that I built Mesa 10.1 from git and installed it on Ubuntu
15.04. Along with Xorg 1.17.1 and the latest DDX compiled from git, I can't
observe the bug.

Is there anything else that I can do to help this along? I tried cloning the
master branch and just reverting Marek's commit (the one that I narrowed the
bug down to with my git bisect), but of course that didn't work since there is
other newer code which now depends on that.

I also tried disabling hyperz (since I believe 10.2 turned hyperz on by
default), and that had no effect.