media: hantro: Stop using H.264 parameter pic_num

The hardware expects FrameNumWrap or long_term_frame_idx. Picture
numbers are per field, and are mostly used during the memory
management process, which is done in userland. This fixes two
ITU conformance tests:

  - MR6_BT_B
  - MR8_BT_B

Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Reviewed-by: Sebastian Fricke <sebastian.fricke@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>