videomode: provide dummy inline functions for !CONFIG_OF

If CONFIG_OF is disabled but we build any driver that tries to
use of_get_display_timing, we get this link error:

ERROR: "of_get_display_timing" [drivers/gpu/drm/rcar-du/rcar-du-drm.ko] undefined!

This adds an empty inline function as an alternative, to avoid changing
each driver using this function when we want to build them without
CONFIG_OF.
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

include/video/of_display_timing.h

@@ -15,9 +15,25 @@ struct display_timings;
 
 #define OF_USE_NATIVE_MODE -1
 
+#ifdef CONFIG_OF
 int of_get_display_timing(struct device_node *np, const char *name,
 		struct display_timing *dt);
 struct display_timings *of_get_display_timings(struct device_node *np);
 int of_display_timings_exist(struct device_node *np);
+#else
+static inline int of_get_display_timing(struct device_node *np, const char *name,
+		struct display_timing *dt)
+{
+	return -ENOSYS;
+}
+static inline struct display_timings *of_get_display_timings(struct device_node *np)
+{
+	return NULL;
+}
+static inline int of_display_timings_exist(struct device_node *np)
+{
+	return -ENOSYS;
+}
+#endif
 
 #endif