drm: rcar-du: Use %p4cc to print 4CC format

Use the %p4cc format specifier to print 4CCs, which will provide a more
readable message than the raw hex value.

Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>