drm: rcar-du: Define macros for the max number of groups, CRTCs and LVDS

Let's avoid magic constants. Beside increasing code readability, it will
also ensure that no location will be forgotten when raising the maximum
number of groups, CRTCs or LVDS encoders

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>