drm/rockchip: add CONFIG_OF dependency for lvds

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: 34cc0aa25456 ("drm/rockchip: Add support for Rockchip Soc LVDS")
Signed-off-by: Arnd Bergmann <[email protected]>
Signed-off-by: Sean Paul <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]