Reference Picture Set lists provide indices of short and long term
reference in DBP array.
Fix Hantro to not do a look up in DBP entries.
Make documentation more clear about it.

[hverkuil: fix typo in commit log]
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Reviewed-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>