There seems to be a kernel race when one disconnects an external
monitor connected to a DisplayPort via a USB-C adapter. The race
results in a connector being reported as connected, but without any
modes supported.

This had the side effect that we tried to set a preferred mode to
the first listed mode, but as no modes were available, we instead tried
to dereference the first element of a NULL array, causing a
segmentation fault.

Mitigate this by skipping adding output if no supported modes are
advertised and the output doesn't support scaling, while moving the
fallback path for calculating a preferred output mode to after possibly
adding the common modes, to avoid the unvolentary NULL dereference.

https://bugzilla.gnome.org/show_bug.cgi?id=789501