_C_, I had the same problem. After an hour of reading through the aticonfig info, I came up with these commands to fix it:
sudo aticonfig --set-dispattrib=dfp9,sizeX:1920
sudo aticonfig --set-dispattrib=dfp9,sizeY:1080
My system hung after entering the second command, but when I rebooted the display finally came up correctly.
dfp9 is the DISPLAYTYPE. If that doesn't work for you, you can query the system for the display type with:
Again, this is what worked for me. Your mileage may vary.
10:58 AM by