media: atomisp: Call media_pipeline_alloc_start() in stream start

Call media_pipeline_alloc_start() from atomisp_start_streaming() to mark
all involved subdevs as busy so that the links cannot be changed through
the media-controller APIs while streaming.

Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>