drm: rcar-du: Map memory through the VSP device

For planes handled by a VSP instance, map the framebuffer memory through
the VSP to ensure proper IOMMU handling.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
[Kieran: Fix infinite loop on fail]
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Acked-by: Mauro Cavalho Chehab <mchehab@s-opensource.com>