When the base videodecoder class re-attempts a negotiation after flush, the
vabasedec `need_negotiation` flag isn't necessarily set to TRUE, because in that
situation the input state hasn't changed.
By always chaining up we are sure that buffer pool negotiation will always be
attempted.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9457>