playbin3: take only playsink locking in a same thread without taking playbin3 locking

Submitted by HoonHee Lee

Link to original bug (#796676)

Description

Dear All
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 ().

Thanks.

Version: 1.14.0