media: camss: vfe: Get line pointer as container of video_out

Simplify getting of the line pointer by using the container_of
macro instead of traversing media controller links.

Signed-off-by: Todor Tomov <todor.tomov@linaro.org>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>