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

drivers/gpu/drm/drm_panel.c

@@ -151,10 +151,13 @@ EXPORT_SYMBOL(drm_panel_detach);
  */
 int drm_panel_prepare(struct drm_panel *panel)
 {
-	if (panel && panel->funcs && panel->funcs->prepare)
+	if (!panel)
+		return -EINVAL;
+
+	if (panel->funcs && panel->funcs->prepare)
 		return panel->funcs->prepare(panel);
 
-	return panel ? -ENOSYS : -EINVAL;
+	return 0;
 }
 EXPORT_SYMBOL(drm_panel_prepare);
 
@@ -171,10 +174,13 @@ EXPORT_SYMBOL(drm_panel_prepare);
  */
 int drm_panel_unprepare(struct drm_panel *panel)
 {
-	if (panel && panel->funcs && panel->funcs->unprepare)
+	if (!panel)
+		return -EINVAL;
+
+	if (panel->funcs && panel->funcs->unprepare)
 		return panel->funcs->unprepare(panel);
 
-	return panel ? -ENOSYS : -EINVAL;
+	return 0;
 }
 EXPORT_SYMBOL(drm_panel_unprepare);
 
@@ -190,10 +196,13 @@ EXPORT_SYMBOL(drm_panel_unprepare);
  */
 int drm_panel_enable(struct drm_panel *panel)
 {
-	if (panel && panel->funcs && panel->funcs->enable)
+	if (!panel)
+		return -EINVAL;
+
+	if (panel->funcs && panel->funcs->enable)
 		return panel->funcs->enable(panel);
 
-	return panel ? -ENOSYS : -EINVAL;
+	return 0;
 }
 EXPORT_SYMBOL(drm_panel_enable);
 
@@ -209,10 +218,13 @@ EXPORT_SYMBOL(drm_panel_enable);
  */
 int drm_panel_disable(struct drm_panel *panel)
 {
-	if (panel && panel->funcs && panel->funcs->disable)
+	if (!panel)
+		return -EINVAL;
+
+	if (panel->funcs && panel->funcs->disable)
 		return panel->funcs->disable(panel);
 
-	return panel ? -ENOSYS : -EINVAL;
+	return 0;
 }
 EXPORT_SYMBOL(drm_panel_disable);
 
@@ -228,10 +240,13 @@ EXPORT_SYMBOL(drm_panel_disable);
  */
 int drm_panel_get_modes(struct drm_panel *panel)
 {
-	if (panel && panel->funcs && panel->funcs->get_modes)
+	if (!panel)
+		return -EINVAL;
+
+	if (panel->funcs && panel->funcs->get_modes)
 		return panel->funcs->get_modes(panel);
 
-	return panel ? -ENOSYS : -EINVAL;
+	return -EOPNOTSUPP;
 }
 EXPORT_SYMBOL(drm_panel_get_modes);