backends/drm: Change the base class of DrmLeaseOutput

At the moment, the DrmLeaseOutput class inherits from the
KWaylandServer::DrmLeaseConnectionV1Interface class. While this works,
it's not a future-proof design. For example, kwin could also lease its
"desktop" outputs in order to let another wayland compositor run
alongside it.

Also, it's a good practice to prefer composition over inheritance.