drm: renesas: rz-du: Drop bpp variable from struct rzg2l_du_format_info

Drop the unused variable bpp from struct rzg2l_du_format_info.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Link: https://lore.kernel.org/r/20250330102357.56010-2-biju.das.jz@bp.renesas.com