drm: rcar-du: Fix rcar_du_crtc structure documentation

The rcar_du_crtc structure index field contains the CRTC hardware index,
not the hardware and software index. Update the documentation
accordingly.

Fixes: 5361cc7f ("drm: rcar-du: Split CRTC handling to support hardware indexing")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>