Thanks for bisecting, but I don't think that commit can be directly responsible
for a GPU hang. Before that commit, the DRI3 code in Mesa would only use one
back buffer for glxgears, which means that the GPU could only start rendering a
new frame after the previous one had finished presenting. Maybe that somehow
prevented the hang.

A possible test for this theory is running

 vblank_mode=0 DRI_PRIME=1 glxgears

with Mesa 12.0.3; does that also trigger the hang?