drm: Constify drm_mode_config atomic helper private pointer

The drm_mode_config helper private field points to a structure of
function pointers that don't need to be modified at runtime. Make it
const.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Acked-by: Brian Starkey <brian.starkey@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170102091613.6310-1-laurent.pinchart@ideasonboard.com
diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
index 5a29978..6cd541d6 100644
--- a/include/drm/drm_mode_config.h
+++ b/include/drm/drm_mode_config.h
@@ -661,7 +661,7 @@
 	/* cursor size */
 	uint32_t cursor_width, cursor_height;
 
-	struct drm_mode_config_helper_funcs *helper_private;
+	const struct drm_mode_config_helper_funcs *helper_private;
 };
 
 void drm_mode_config_init(struct drm_device *dev);