backends/drm: Use DrmOutput to represent non-desktop outputs too

The main motivation behind this change is to make the drm backend
multi-purpose. That's it, to make it suitable for implementing all kinds
of compositors. At the moment, there's an artificial split between
"desktop" and "non-desktop" outputs, i.e. VR headsets, which stands in
the way of that and moving the remaining wayland code out of the drm
backend for better layering and architecture.