composition: Avoid deadlock when setting current bin to ready (on commit or seek)

composition: Avoid deadlock when setting current bin to ready (on commit or seek)

We need to make sure that between the time we send flush_start/stop and
the time we actually set the bin to READY, no buffer got prerolled again
as it would lead to a deadlock trying to set the bin to READY (while
deactivating the pads, it needs the streaming lock, which would be
taken in that case)

Co-Authored by: Mathieu Duponchelle <mathieu.duponchelle@opencreed.com>