drm: rcar-du: Fix DIDSR field name

The DIDSR fields named LDCS were incorrectly defined as LCDS.
Both the Gen2 and Gen3 documentation refer to the fields as the "LVDS
Dot Clock Select".

Correct the definitions.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>