drm/rockchip: add CONFIG_OF dependency for lvds

Build-testing on randconfig kernels revealed a dependency in the
newly added lvds sub-driver:

drivers/gpu/drm/rockchip/rockchip_lvds.c: In function 'rockchip_lvds_bind':
drivers/gpu/drm/rockchip/rockchip_lvds.c:380:24: error: 'struct drm_bridge' has no member named 'of_node'
   remote = lvds->bridge->of_node;

We could work around that in the code, adding a Kconfig dependency
seems easier.

Fixes: 34cc0aa2 ("drm/rockchip: Add support for Rockchip Soc LVDS")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20171106135852.1355487-1-arnd@arndb.de