media: atomisp: Make atomisp_q_video_buffers_to_css() static

atomisp_q_video_buffers_to_css() is only used insidd atomisp_fops.c, make
it static.

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>