bgo#670459 [GnomeRR] - Consider Embedded Display Port outputs as the laptop's built-in display

xorg still hasn't fixed https://bugs.freedesktop.org/show_bug.cgi?id=26736 which is
about drivers properly exposing the ConnectorType property.  This would let us
unambiguously identify a laptop's built-in display without playing heuristics
on the RANDR output names.  For now, we'll use 'eDP' as the name given to
Embedded Display Port outputs.