media: bcm2835-unicam: Start and stop media_pipeline with same node

media_pipeline_start and media_pipeline_stop now validate that
the pipeline is being started and stopped with the same pipe
and pad handles.
When running with embedded metadata (eg imx477 and imx708), the
start typically happens from the metadata pad, whilst stop is
always from the image pad.

Always pass the image pad to media_pipeline_start to ensure
that the calls are balanced.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>