Comment 90 for bug 537640

The modeset command fixed this issue for me. I saw a comment above about 'how to'. It's pretty simple:

sudo vi /etc/modprobe.d/radeon-kms.conf

and add the following line (press 'o' for Open Line, type in the text, press Esc and then ':wq' for command-write-quit, the colon is important!)

options radeon modeset=0