logind: ignore lid switch if more than 1 display is connected

logind: ignore lid switch if more than 1 display is connected

Previously we expected the desktop environment to take an inhibitor
lock, but this opened a race on boot-up where logind might already be
running but no DE is active.

Hence, let's move checking for additional displays into logind. This
also opens up this logic for other DEs, given that only GNOME
implemented the inhibitor logic so far.