Comment # 1 on bug 903544 from Adriano Argiolas

Issue resolved by adding:

Option         "RegistryDwords"         
"EnableBrightnessControl=1;PowerMizerEnable=0x1;PerfLevelSrc=0x3333;PowerMizerLevel=0x3;PowerMizerDefault=0x3;PowerMizerDefaultAC=0x3"

to the Device section of Xorg.conf