media: rkisp1: isp: Disallow multiple active sources

The ISP supports multiple source subdevs, but can only capture from a
single one at a time. The source is selected through link setup. The
driver finds the active source in its .s_stream() handler using the
media_entity_remote_pad() function. This fails to reject invalid
configurations with multiple active sources. Fix it by using the
media_pad_remote_pad_unique() helper instead, and inline
rkisp1_isp_get_source() in rkisp1_isp_s_stream() as the function is
small and has a single caller.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Dafna Hirschfeld <dafna@fastmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>