Now that mesa 9.1 is finally in raring, it is not necessary to compile mesa, even if 9.2 is probably better for radeonsi.
--enable-gbm was probably never needed, as it is already enabled by dependencies.
That leaves installing glamor, recompiling xserver-xorg-video-ati with glamor, and configuring xorg.conf
I have made a stab at packaging this at https://launchpad.net/~cybjit/+archive/ppa-glamor/.
After installing xserver-xorg-video-glamor and upgrading xserver-xorg-video-radeon, only configuring xorg.conf should be needed.