drm: rcar-du: lvds: Fix LVDCR1 for R-Car gen3

The LVDCR1 register for the R-Car gen3 SoCs was documented as having the
layout different from the gen2 SoCs in  the early R-Car gen3 manuals but
since v0.52 the LVDCR1 layout is described as being the same as on the gen2
SoCs; the old CHn control values are said to be prohibited now (and there
seems to be no valid output signal when they are used).

Fixes: 6bc2e15c ("drm: rcar-du: lvds: Add R-Car Gen3 support")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>