drm: rcar-du: Use DRM-managed allocation for VSP planes

devm_kcalloc() is the wrong API to allocate planes, as the lifetime of
the planes is tied to the DRM device, not the device to driver
binding. drmm_kcalloc() isn't a good option either, as it would result
in the planes being freed before being unregistered during the managed
cleanup of the DRM objects. Use a plain kcalloc(), and cleanup the
planes and free the memory in the existing rcar_du_vsp_cleanup()
handler.

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