Send only current wl_output mode

Non-current output modes were deprecated due to various reasons, e.g.
it's not possible to remove some nodes; two modes can have the same
resolution and the refresh rate but different flags, but wl_output.mode
fails to communicate that properly; the usefulness of non-current modes
is questionable. For more details, please refer to [1].

[1] https://gitlab.freedesktop.org/wayland/wayland/-/issues/92