A solução:
sudo apt-get remove --purge fglrx*
sudo apt-get remove --purge xserver-xorg-video-ati xserver-xorg-video-radeon
sudo apt-get install --reinstall xserver-xorg-core libgl1-mesa-glx libgl1-mesa-dri
sudo rm /etc/X11/xorg.conf.o* /etc/X11/xorg.conf.0* /etc/X11/xorg.conf.f*
sudo rm -rf /etc/ati
sudo apt-get install --reinstall ubuntu-desktop