drm: rcar-du: Don't update planes on disabled CRTCs

A disabled CRTC can't display planes, the driver shouldn't try to
configure it when updating planes. The DRM core will store the plane
state for us, and the plane will be configured appropriately the next
time the CRTC is enabled.

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