How do I reinstall the default (non-proprietary) display drivers?

Solution 1:

To properly revert the graphic drivers to the out of the box state do the following commands in a terminal.

sudo apt-get remove --purge xorg-driver-fglrx fglrx*
sudo apt-get remove --purge fglrx*
sudo apt-get install build-essential cdbs fakeroot dh-make debhelper debconf libstdc++6 dkms libqtgui4 wget execstack libelfg0 dh-modaliases
sudo apt-get install --reinstall libgl1-mesa-glx libgl1-mesa-dri
sudo mv /etc/X11/xorg.conf /etc/X11/xorg.conf.old
sudo dpkg-reconfigure xserver-xorg

Now reboot your system and everything will be fine.

Solution 2:

You would simply uninstall the display driver you have installed. Open jockey-gtk aka "Additional drivers", click on the activated driver and click remove (lower right corner). This should revert the settings to your former ones.