drm: rcar-du: Split planes pre-association 4/4 between CRTCs

If we have more than one CRTCs in a group pre-associate planes 0-3 with
CRTC 0 and planes 4-7 with CRTC 1 to minimize flicker occurring when the
association is changed. The pre-association could be controlled by a
module parameter if needed.

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