media: v4l: vsp1: Share duplicated DRM pipeline configuration code

Move the duplicated DRM pipeline configuration code to a function and
call it from vsp1_du_setup_lif() and vsp1_du_atomic_flush().

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>