If a controller (usually, eDP!) does not support audio, or audio is not
enabled because the endpoint has no audio support, it's useless to lock
a mutex only to unlock it right after because there's no .plugged_cb().
Check if the audio is supported and enabled before locking the mutex in
mtk_dp_update_plugged_status(): if not, we simply return immediately.
While at it, since the update_plugged_status_lock mutex would not be
used if the controller doesn't support audio at all, initialize it
only if `audio_supported` is true.