video: Add video_is_active function

Add the helper function video_is_active() to test if one video device
is active.

This function can be used in board code to execute operation
only when the display is probed / really used.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>

include/video.h

@@ -276,6 +276,13 @@ static inline int video_sync_copy_all(struct udevice *dev)
 
 #endif
 
+/**
+ * video_is_active() - Test if one video device it active
+ *
+ * @return true if at least one video device is active, else false.
+ */
+bool video_is_active(void);
+
 #ifndef CONFIG_DM_VIDEO
 
 /* Video functions */