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.