media: vsp1: Rename dl_child to dl_next

Both vsp1_dl_list_commit() and __vsp1_dl_list_put() walk the display
list chain referencing the nodes as children, when in reality they are
siblings.

Update the terminology to 'dl_next' to be consistent with the
vsp1_video_pipeline_run() usage.

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