drm/fsl-dcu: rework codes to support of_graph dt binding for panel

This patch rework the output code to add of_graph dt binding support
for panel device and also keeps the backward compatibility

Signed-off-by: Meng Yi <meng.yi@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stefan Agner <stefan@agner.ch>