media: hantro: Move H264 motion vector calculation to a helper

Move the extra bytes calculation that are needed for H264
motion vector to a helper. This is just a cosmetic cleanup.

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>