v4l: vsp1: Fill display list headers without holding dlm spinlock

The display list headers are filled using information from the display
list only. Lower the display list manager spinlock contention by filling
the headers without holding the lock.

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