backends/drm: Allow leasing all outputs

At the moment, we allow leasing only non-desktop outputs, i.e. VR headset outputs, but it also makes sense to allow leasing normal desktop outputs. For example, in order to run gamescope on one output in plasma wayland session.

This change allows leasing all outputs in the drm backend. It would be great to get rid of some code duplication and use the same type to represent both non-desktop and desktop outputs. I plan to do this refactoring later.