drm_modes: add drm_display_mode_to_videomode

Add conversion from drm_display_mode to videomode.

Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
diff --git a/include/drm/drm_modes.h b/include/drm/drm_modes.h
index 91d0582..321ae64 100644
--- a/include/drm/drm_modes.h
+++ b/include/drm/drm_modes.h
@@ -197,6 +197,8 @@
 					      int GTF_K, int GTF_2J);
 void drm_display_mode_from_videomode(const struct videomode *vm,
 				     struct drm_display_mode *dmode);
+void drm_display_mode_to_videomode(const struct drm_display_mode *dmode,
+				   struct videomode *vm);
 int of_get_drm_display_mode(struct device_node *np,
 			    struct drm_display_mode *dmode,
 			    int index);