Delay start-up until a graphics device is present

I just learned about https://gitlab.gnome.org/GNOME/gdm/issues/103 and I got the feeling that solving the same issue in Weston might be beneficial to those who run Weston as a system service.

In short, when using logind, check the specific property, and if it is false, delay initialization of the DRM backend until it changes to true.

I think this is a well-defined and fairly isolated task, so I marked it as good for new contributors.

However, this is very low priority as I don't recall anyone asking for this feature.