avviddec: Report a latency even without upstream framerate

avviddec: Report a latency even without upstream framerate

There are cases where upstream will not provide a framerate, or it won't be
fixed. But if there is latency introduced by the decoder we do want to report
it.

Therefore use the framerate stored in the actual decoder, which will have a
default.

Fixes hangs when playing back such streams with decodebin3 (where the multiqueue
will not have been informed of that downstream latency and not grow accordingly)

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3391>