media: hantro: postproc: Properly calculate chroma offset

Currently chroma offset calculation assumes only 1 byte per luma, with
no consideration for stride.

Take necessary information from destination pixel format which makes
calculation completely universal.

Reviewed-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Tested-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>