drm_modes: add of_videomode helpers

Add helper to get drm_display_mode from devicetree.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Reviewed-by: Thierry Reding <thierry.reding@avionic-design.de>
Acked-by: Thierry Reding <thierry.reding@avionic-design.de>
Tested-by: Thierry Reding <thierry.reding@avionic-design.de>
Tested-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Afzal Mohammed <Afzal@ti.com>
Tested-by: Rob Clark <robclark@gmail.com>
Tested-by: Leela Krishna Amudala <leelakrishna.a@gmail.com>
diff --git a/include/drm/drmP.h b/include/drm/drmP.h
index d5c06ff..fcc9d23 100644
--- a/include/drm/drmP.h
+++ b/include/drm/drmP.h
@@ -85,6 +85,7 @@
 struct drm_file;
 struct drm_device;
 
+struct device_node;
 struct videomode;
 
 #include <drm/drm_os_linux.h>
@@ -1460,6 +1461,9 @@
 
 extern int drm_display_mode_from_videomode(const struct videomode *vm,
 					   struct drm_display_mode *dmode);
+extern int of_get_drm_display_mode(struct device_node *np,
+				   struct drm_display_mode *dmode,
+				   int index);
 
 /* Modesetting support */
 extern void drm_vblank_pre_modeset(struct drm_device *dev, int crtc);