drm: rcar-du: lvds: Refactor LVDS startup

After the recent corrections to the R-Car gen2/3 LVDS startup code, already
similar enough at their ends rcar_lvds_enable_gen{2|3}() started asking for
a merge and it's becoming actually necessary with the addition of the R-Car
V3M (R8A77970) support -- this gen3 SoC has gen2-like LVDPLLCR layout.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
[Set the LVDS mode and input before turning channels on]
[Rebased, coding style changes]
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>