Comment # 5 on bug 59015 from Alex Deucher

I'm assuming you are using mesa 9.x or newer based on the version info in the
bug header.  Can you confirm?  xf86-video-ati 7.0.0 requires mesa 9.x.  Also,
if the steam client is a 32-bit application and you are running a 64-bit
distro, you will need to make sure your 32-bit 3D driver is up to date.  I
suspect you have an out of date 32-bit 3D driver.