drm/drm_panel: no error when no callback

The callbacks in drm_panel_funcs are optional, so do not
return an error just because no callback is assigned.

v2:
- Document what functions in drm_panel_funcs are optional (Laurent)
- Return -EOPNOTSUPP if get_modes() is not assigned (Laurent)
  (Sam: -EOPNOTSUPP seems to best error code in this situation)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191207140353.23967-2-sam@ravnborg.org

include/drm/drm_panel.h

@@ -65,6 +65,8 @@ struct drm_panel_funcs {
 	 * @prepare:
 	 *
 	 * Turn on panel and perform set up.
+	 *
+	 * This function is optional.
 	 */
 	int (*prepare)(struct drm_panel *panel);
 
@@ -72,6 +74,8 @@ struct drm_panel_funcs {
 	 * @enable:
 	 *
 	 * Enable panel (turn on back light, etc.).
+	 *
+	 * This function is optional.
 	 */
 	int (*enable)(struct drm_panel *panel);
 
@@ -79,6 +83,8 @@ struct drm_panel_funcs {
 	 * @disable:
 	 *
 	 * Disable panel (turn off back light, etc.).
+	 *
+	 * This function is optional.
 	 */
 	int (*disable)(struct drm_panel *panel);
 
@@ -86,14 +92,18 @@ struct drm_panel_funcs {
 	 * @unprepare:
 	 *
 	 * Turn off panel.
+	 *
+	 * This function is optional.
 	 */
 	int (*unprepare)(struct drm_panel *panel);
 
 	/**
 	 * @get_modes:
 	 *
-	 * Add modes to the connector that the panel is attached to and
-	 * return the number of modes added.
+	 * Add modes to the connector that the panel is attached to
+	 * and returns the number of modes added.
+	 *
+	 * This function is mandatory.
 	 */
 	int (*get_modes)(struct drm_panel *panel);
 
@@ -102,6 +112,8 @@ struct drm_panel_funcs {
 	 *
 	 * Copy display timings into the provided array and return
 	 * the number of display timings available.
+	 *
+	 * This function is optional.
 	 */
 	int (*get_timings)(struct drm_panel *panel, unsigned int num_timings,
 			   struct display_timing *timings);