playbin3: take only playsink locking in a same thread without taking playbin3 locking
Submitted by HoonHee Lee
I have observed deadlock problem in our target platform with gst v1.14.
I think the deadlock is happened between getting property 'PROP_SUBTITLE_ENCODING' and activate_chain ().
I think it makes sense not to taking playbin3 locking with playsink locking in a same thread in gst_play_sink_get_subtitle_encoding ().