Add a linear pipeline logic for the stream control. It's created by
walking backwards on the entity graph. When the stream starts it will
simply loop through the pipeline calling the respective process_frame
function of each entity.

Fixes: f2fe8906 ("vimc: Virtual Media Controller core, capture
and sensor")

Cc: stable@vger.kernel.org # for v4.20
Signed-off-by: Lucas A. M. Magalhães <lucmaga@gmail.com>
Acked-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil-cisco@xs4all.nl: fixed small space-after-tab issue in the patch]
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>