https://bugs.freedesktop.org/show_bug.cgi?id=98146
--- Comment #6 from Mike Lothian mike@fireburn.co.uk --- 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