media: stm32: dcmi: only call s_stream on the source subdev

Avoid calling s_stream on each subdev until reaching the sensor and
instead call s_stream on the source subdev only (which will in turn
do whatever needed to start the stream).

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Reviewed-by: Hugues FRUCHET <hugues.fruchet@foss.st.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>