I applied the patches described at https://bugs.freedesktop.org/show_bug.cgi?id=3D43448 to the source packages libdrm 2.4.33-1, mesa 8.0.3-1 and xserver-xorg-video-ati 6.14.4-4. The X server now starts successfully and the on-screen font rendering corruptions are gone. It appears that this is a working solution, until the next upstream upgrade comes along.