drm: rcar-du: Fix crash when enabling a non-visible plane

The DU driver handles non-visible planes (fully clipped by the display's
boundaries) by considering them as disabled. It thus disables the plane
at the hardware level when the plane is moved off-screen. However, if
the plane was previously disabled and is non-visible when it gets
enabled, the attempt to disable it crashes, as the plane wasn't
previously enabled. Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>