media: hantro: Set buffers' zeroth plane payload in .buf_prepare

Buffers' zeroth plane payload size is calculated at format
negotiation time, and so it can be set in .buf_prepare.

Keep in mind that, to make this change easier, hantro_buf_prepare
is refactored, using the cedrus driver as reference. This results
in cleaner code as byproduct.

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>