decklinkvideosink: Only start if both sinks are set to PLAYING already

Not only if the video sink is set to PLAYING so far. Also give more
useful debug output about why we don't start, and don't start if already
started.

Also refactor the function to early-return instead of having a huge
if-else block over the whole function.

https://bugzilla.gnome.org/show_bug.cgi?id=790114
1 file changed