Kconfig fix for RZ/G2L DSI
Signed-off-by: Dave Airlie <airlied@redhat.com>

From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/Y3wYk/Bn/qVa9ha0@pendragon.ideasonboard.com