https://bugs.freedesktop.org/show_bug.cgi?id=104963
--- Comment #3 from Will bstarms@hotmail.com --- Chiming in to say I've had the same issue for an extremely frustratingly long time. I haven't been able to run anything newer than 14.04 on this machine until today. I can confirm that the radeon.dpm setting doesn't seem to matter, it's nomodeset that lets me boot.
I've got the same motherboard, an MSI A88XM-345 on the latest (hah, 2014, E7721AMS V25.6) BIOS, and an A8-6600K APU.