media: h264: Use v4l2_h264_reference for reflist

In preparation for adding field decoding support, convert the byte arrays
for reflist into array of struct v4l2_h264_reference. That struct will
allow us to mark which field of the reference picture is being referenced.

[hverkuil: top_field_order_cnt -> pic_order_count]

Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Reviewed-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>