decodebin2: Avoid deactivation races

decodebin2: Avoid deactivation races

Deactivating pads from two threads isn't 100% MT-safe. There is a
slim chance that the GstPadActivateFunc might be called twice with
the same values (in this case from the cleanup thread *and* from
the GstElement change_state function when going from PAUSED to READY).

In order to avoid that, call any existing cleanup function *before*
calling the parent change_state implementation on downwards state
changes.