drm: rcar-du: rzg2l_mipi_dsi: Enhance device lanes check

Enhance device lanes check by reading TXSETR register at probe(),
and enforced in rzg2l_mipi_dsi_host_attach().

As per HW manual, we can read TXSETR register only after
DPHY initialization.

Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>