Video is broken with switchable graphics

Originally created by @alant on #7505 (Redmine)

On a MacBook Pro 6,2, which has both an intel and nvidia graphics card Tails 1.1-rc1 boots fine but X fails to start.

It is possible to make it work by booting with i915.modeset=0 rootpw=pass and drop a file containing:

Section "Device"
    Identifier "Device0"
    Driver "nouveau"
    BusID "1:0:0"
EndSection

and restart X with service gdm3 restart

Feature Branch: bugfix/7505-optimus

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