lavfi: fix filter format negotiation loop.

lavfi: fix filter format negotiation loop.

query_formats() returning EAGAIN is not considered a progress
in the format negotiation.

If the filter returns EAGAIN but did set some of its formats
lists, it could be considered a partial success and counted
as progress in the negotiation. Not counting it is not a
problem because it currently only happens in the first round,
where there will always be some progress on the other filters.