video: Track whether a display is in use

Mark a display as in use when display_enable() is called. This can avoid
a display being used by multiple video-output devices.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/display.h b/include/display.h
index b1c4766..d0a08d4 100644
--- a/include/display.h
+++ b/include/display.h
@@ -16,10 +16,12 @@
  * @source_id:	ID for the source of the display data, typically a video
  * controller
  * @src_dev:	Source device providing the video
+ * @in_use:	Display is being used
  */
 struct display_plat {
 	int source_id;
 	struct udevice *src_dev;
+	bool in_use;
 };
 
 /**
@@ -41,6 +43,14 @@
 int display_enable(struct udevice *dev, int panel_bpp,
 		   const struct display_timing *timing);
 
+/**
+ * display_in_use() - Check if a display is in use by any device
+ *
+ * @return true if the device is in use (display_enable() has been called
+ * successfully), else false
+ */
+bool display_in_use(struct udevice *dev);
+
 struct dm_display_ops {
 	/**
 	 * read_timing() - Read information directly