drm/rockchip: add PINCTRL dependency for LVDS

The new driver fails to build when CONFIG_PINCTRL is disabled:

drivers/gpu/drm/rockchip/rockchip_lvds.c: In function 'rockchip_lvds_grf_config':
drivers/gpu/drm/rockchip/rockchip_lvds.c:229:39: error: dereferencing pointer to incomplete type 'struct dev_pin_info'
   if (lvds->pins && !IS_ERR(lvds->pins->default_state))

This adds the respective Kconfig dependency.

Fixes: 34cc0aa2 ("drm/rockchip: Add support for Rockchip Soc LVDS")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Mark Yao <mark.yao@rock-chips.com>
Signed-off-by: Mark Yao <mark.yao@rock-chips.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171005120957.485433-1-arnd@arndb.de