drm/modes: add non-OF stub for of_get_drm_display_mode

If CONFIG_OF is disabled, of_get_drm_display_mode is not compiled in,
and drivers using it fail to build:

  ld: drivers/gpu/drm/imx/parallel-display.o: in function `imx_pd_connector_get_modes':
  parallel-display.c:(.text+0x8d): undefined reference to `of_get_drm_display_mode'

Add an inline stub so they can be build-tested with non-OF
configurations.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

include/drm/drm_modes.h

@@ -461,9 +461,19 @@ void drm_display_mode_from_videomode(const struct videomode *vm,
 void drm_display_mode_to_videomode(const struct drm_display_mode *dmode,
 				   struct videomode *vm);
 void drm_bus_flags_from_videomode(const struct videomode *vm, u32 *bus_flags);
+
+#if defined(CONFIG_OF)
 int of_get_drm_display_mode(struct device_node *np,
 			    struct drm_display_mode *dmode, u32 *bus_flags,
 			    int index);
+#else
+static inline int of_get_drm_display_mode(struct device_node *np,
+					  struct drm_display_mode *dmode,
+					  u32 *bus_flags, int index)
+{
+	return -EINVAL;
+}
+#endif
 
 void drm_mode_set_name(struct drm_display_mode *mode);
 int drm_mode_vrefresh(const struct drm_display_mode *mode);