drm/rockchip: rgb: add stub functions when rgb encoder is disabled

The newly added internal rgb encoder for Rockchip vops is missing
stubs for the case that the rgb output part is not enabled in the
kernel config. So add these.

Fixes: 1f0f01515172 (drm/rockchip: Add support for Rockchip Soc RGB output interface)
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
[seanpaul fixed up checkpatch nits]
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180905191302.26023-1-heiko@sntech.de

drivers/gpu/drm/rockchip/rockchip_rgb.h

@@ -14,7 +14,20 @@
  * GNU General Public License for more details.
  */
 
+#ifdef CONFIG_ROCKCHIP_RGB
 struct rockchip_rgb *rockchip_rgb_init(struct device *dev,
 				       struct drm_crtc *crtc,
 				       struct drm_device *drm_dev);
 void rockchip_rgb_fini(struct rockchip_rgb *rgb);
+#else
+static inline struct rockchip_rgb *rockchip_rgb_init(struct device *dev,
+						     struct drm_crtc *crtc,
+						     struct drm_device *drm_dev)
+{
+	return NULL;
+}
+
+static inline void rockchip_rgb_fini(struct rockchip_rgb *rgb)
+{
+}
+#endif