wayland: Add wl_drm listener immediately upon bind

wayland: Add wl_drm listener immediately upon bind

Events on wl_drm could get lost if the listener is bound only after
the roundtrip is complete. While it is correct that to get all wl_drm
events at most two roundtrips are required, in fact one roundtrip may
be enough if another thread flushes and reads from the connection
in parallel to the initial libva wl_registry roundtrip. It is thus too
late to call wl_drm_add_listener after the roundtrip.

Fixes: #101