drm: rcar-du: Keep plane to CRTC associations when disabling a plane

Changing the plane to CRTC associations requires restarting the CRTC
group, creating visible flicker. Mitigate the issue by changing plane
association only when a plane becomes enabled, not when it get disabled.

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