omxvideodec: always signal drain cond when stopping streaming loop

omxvideodec: always signal drain cond when stopping streaming loop

If for some reason something goes wrong and we stop the streaming loop
we may end up with other threads still waiting on the drain cond.
No more buffers will be produced by the component so they were waiting
forever.

Fix this by always signalling this cond when stopping the streaming
loop.

https://bugzilla.gnome.org/show_bug.cgi?id=796207