drm: rcar-du: Use NULL for table initialisation

Replace the initialisation of the vsps table with a NULL specifier.

Fixes the following warning:
 linux/drivers/gpu/drm/rcar-du/rcar_du_kms.c:483:40:
    warning: Using plain integer as NULL pointer
      CC      drivers/gpu/drm/rcar-du/rcar_du_kms.o

Fixes: 3e81374e ("drm: rcar-du: Support multiple sources from the same VSP")
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Vaishali Thakkar <vthakkar@vaishalithakkar.in>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>