wayland/output: Always set xdg_output interface implementation

This avoids the following crash, that could happen in certain rare race conditions, e.g. in tests:

0. wl_closure_invoke (closure=0x2fbf9e0, target=0x2e5b3d0, opcode=0) at ../src/connection.c:1014
1. wl_client_connection_data () at ../src/wayland-server.c:432
2. wl_event_loop_dispatch () at ../src/event-loop.c:1027
3. wayland_event_source_dispatch () at ../src/wayland/meta-wayland.c:104
4. g_main_dispatch () at ../glib/gmain.c:3381