Comment 8 for bug 773734

FWIW: Stephane helped me find out that you can forcefully disconnect the eDP1 output via a kernel boot parameter. We added video=eDP-1:d as a kernel boot param (manually during bootup, or directly by adding it in /boot/grub/grub.cfg).
In my case it doesn't seem to help much: eDP1 output appears indeed "disconnected" in xrandr, but unsurprisingly LVDS1 does not want to turn on anymore (as they seem to be linked somehow). And as a bonus side effect, the VGA1 output does not work properly in 1680x1050 anymore, instead it has a bottom left area of 1024x768 that works, and the other 3 quarters of the screen are black or show graphical glitches... not usable.