bgo#672030 - Use "default" as the laptop's built-in output's name as a...

bgo#672030 - Use "default" as the laptop's built-in output's name as a catch-all for NVidia and others

Drivers that don't bother to do RANDR properly tend to use "default" for the built-in
output's name.  So, let's use that as a last heuristic to detect laptop displays.

Patch by James M. Leddy.
Original bug at https://launchpad.net/bugs/949296
(cherry picked from commit cc326c09)