I've git bisected from git master and came up with this commit that causes GPU
to lock up (DRI driver locks up, I can ssh an reboot the laptop cleanly
however).

Kernel: 3.17.2-300.fc21.x86_64

[root@devbox mesa-20141127]# git bisect good
7b4276d7acf2e0f77044cb50caa6ad936fa78786 is the first bad commit
commit 7b4276d7acf2e0f77044cb50caa6ad936fa78786
Author: Michel Dänzer <michel.daenzer@amd.com>
Date:   Tue Aug 26 18:21:50 2014 +0900

    r600g,radeonsi: Always use GTT again for PIPE_USAGE_STREAM buffers

    Putting those in VRAM can cause long pauses due to buffers being moved
    into / out of VRAM.

    Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=84662
    Cc: mesa-stable@lists.freedesktop.org
    Reviewed-by: Alex Deucher <alexander.deucher@amd.com>

:040000 040000 672dc7643603cfccc8b9c85c83f37db32c26beeb
8e4a9f33de638b7df98284614a0da3bb3373f320 M      src

With this reverted from this point back to Sept 7th, I have stability with the
r600g DRI driver.