This field is not used anymore. This also gives us the opportunity to clean up backend autocreation.
We can grab the event loop from the wlr_session instead.
wl_display holds a lot more than wlr_session needs: wlr_session only needs to wait for a FD to become readable, but wl_display provides full access to the Wayland client and protocol objects. Switch to wl_event_loop to better reflect the above.
This helper is responsible for listening for new DRM devices and create new child DRM backends as necessary.
