I had the same issue trying to play CSS.
I got the latest driver from AMD here http://support.amd.com/us/gpudownload/linux/Pages/radeon_linux.aspx?type=2.4.1&product=220.127.116.11.42&lang=English
and installed it but then got the error so I removed it using this command
I changed to the usr/share/ati folder and opened a terminal there and ran
sudo sh amd-uninstall.sh
then I went to menu > preference > software sources - additional drivers tab and selected - video driver AMD fglrx-updates and activated that driver -- this I think install something missing from AMD's driver above
then go back menu > preference > software sources - additional drivers and changed back to - using xorg xserver.
reinstall AMD driver from AMD 's website above. This worked for me. Except for one slight problem game launches but black/blank screen either set it to -windowed launch option within steam or once the game has launched press
ctrl+alt+f1 and then alt+f8 this switches out to console and back redraws the opengl screen I think and the picture comes back
ps im running Linux Mint Cinnamon 64 bit version oh and game performance is excellent