Send kde_output_device_v1.current_mode to the right resource

Currently, send_current_mode() is called for every mode resource, no
matter whether the given output resource and mode resource belong to the
same client, libwayland-server doesn't like that and it prints a warning
about a compositor bug.

With this change, the output device will remember the mode resources for
every output resource and avoid sending current_mode events with modes
that belong to other clients.