Try the instructions in Matthew's post :
http://www.phoronix.com/forums/showpost.php?p=36734&postcount=40
Probably worth reading the whole thread to understand earlier solutions and common typos.
BTW the issue is that many TV/monitor displays have a default "overscan" setting which is fine for TV but really bad for a monitor since you can't see important little things like the menu bar. The driver defaults to a "safe" zoomed out display on HDMI-attached HD TVs to make sure that you can see the entire screen even if the TV defaults are set to overscan, but for now you need to run an aticonfig command to get rid of the safe mode.
EDIT - in the interest of efficiency, here is Matthew's post