drm: rcar-du: Fix rcar_du_of_init() stub

The rcar_du_of_init() function is supposed to be defined as a stub when
CONFIG_DRM_RCAR_LVDS is disabled as the rcar_du_of.c file isn't compiled
in that case. However, a bug in the configuration option check makes it
a stub when CONFIG_DRM_RCAR_LVDS=m as well, which prevents legacy DTs
from being fixed at boot time. Fix the configuration option check by
using IS_ENABLED.

Fixes: 81c0e3dd ("drm: rcar-du: Fix legacy DT to create LVDS encoder nodes")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180515155736.3379-1-laurent.pinchart+renesas@ideasonboard.com