Comment # 6 on bug 98146 from Mike Lothian

An easy way to check it to see if DRI_PRIME=1 glxinfo works without issuing any
xrandr commands

I don't have individual snippits for the drivers only this:

Section "Device" 
        Identifier      "Graphics"
        Option          "DRI"   "3"
EndSection

Also give it a go with the modesetting driver