h264: Fix direct temporal mvs for bottom-field-first poc order

Such files can be created using the --bff x264 option.

Sample-Id: h264_direct_temporal_mvs_bff.mkv
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
