flacenc: Pass audio info from set_format() to query_total_samples() explicitly

This fixes writing of the seek table header.

gst_audio_encoder_get_audio_info() will still return old/unset audio
info until set_format() has actually returned, which then results in
query_total_samples() to always return 0.

Thanks to Jacob Kauffmann for debugging this and finding the main cause.

Fixes #756

Part-of: <!661>