ui/dbus: tear down clipboard callbacks on display finalize

The clipboard D-Bus teardown path currently runs when the peer
disappears, but not when DBusDisplay itself is finalized.

That leaves pending clipboard requests and signal handlers associated
with the clipboard proxy active past display teardown.

Add an explicit clipboard fini hook and invoke it from
dbus_display_finalize() so the clipboard teardown also runs during
display destruction.

bixes: ff1a5810f61f ("ui/dbus: add clipboard interface")
Signed-off-by: GuoHan Zhao <zhaoguohan@kylinos.cn>
Message-ID: <20260330091310.42868-1-zhaoguohan@kylinos.cn>
[ Marc-André - Move clipobard finalization to the function]
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>