media: v4l2-core: Add helpers to build the H264 P/B0/B1 reflists

Building those list is a standard procedure described in section
'8.2.4 Decoding process for reference picture lists construction' of
the H264 specification.

We already have 2 drivers needing the same logic (hantro and rkvdec) and
I suspect we will soon have more.

Let's provide generic helpers to create those lists.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>