media: rkisp1: isp: Add rkisp1_device backpointer to rkisp1_isp

The rkisp1_isp structure documentation mentions a backpointer field to
rkisp1_device, but the field is missing. Add it, and use it to replace
more complicated constructs using container_of() on the v4l2_device.

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