drm/panel: add kernel doc for size attributes in panel_desc

Document that 'width' and 'height' are measured in millimeters.
Signed-off-by: Ulrich Ölmann <u.oelmann@pengutronix.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/gpu/drm/panel/panel-simple.c

@@ -44,6 +44,10 @@ struct panel_desc {
 
 	unsigned int bpc;
 
+	/**
+	 * @width: width (in millimeters) of the panel's active display area
+	 * @height: height (in millimeters) of the panel's active display area
+	 */
 	struct {
 		unsigned int width;
 		unsigned int height;