wayland: Partially revert 797ccca4
If an output is turned off, the client will receive a leave event. In that case, many applications will re-compute the new buffer scale and provide a new buffer.
If the dpms mode changes, for example due to being idle for too long, clients should ideally do nothing, i.e. don't repaint their surfaces, etc.
If an output is disabled for good, the corresponding wl_output will be removed, and the clients will receive wl_surface::leave prior to that.