[media] v4l: vsp1: Remove redundant pipe->dl usage from drm

The pipe->dl is used only inside vsp1_du_atomic_flush(), and can be
obtained and stored locally to simplify the code.

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