If someone has some time to play with this stuff and wants to (try to) figure out where it broke:
git clone git://anongit.freedesktop.org/git/mesa/drm
git bisect start
git checkout libdrm-2.4.56
git bisect good
git checkout libdrm-2.4.58
git bisect bad
At that point, you'll get a different commit to build and test. What I usually do is something like this:
./autogen.sh
./configure (options from build script)
make
make install DESTDIR=/tmp/testing
*open a new root terminal window*
cd /tmp/testing
chown -R root:root .
rm -rf ./usr/man ./usr/doc ./usr/share/doc
makepkg -c n -l y /tmp/libdrm-testing1-$arch-1.tgz
Upgrade to that package, test it (no reboot needed - just a kill/restart of X, I would think), and then go back into the libdrm git directory. If all is well with that package, then do this:
git bisect good
If not, then do this:
git bisect bad
Either way, you'll get a new revision to test. You'll need to clean out the leftovers from the prior build first though:
make clean
git clean
Now repeat the build/install process and loop until you find the bad commit.