drm: renesas: rz-du: rzg2l_du_encoder: Fix max dot clock for DPI

As per the RZ/G2UL hardware manual Table 33.4 Clock List, the maximum
dot clock for the DPI interface is 83.5 MHz. Add mode_valid callback
to reject modes greater than 83.5 MHz.

Suggested-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241022082433.32513-3-biju.das.jz@bp.renesas.com