drm/panel: do not return negative error codes from drm_panel_get_modes()

None of the callers of drm_panel_get_modes() expect it to return
negative error codes. Either they propagate the return value in their
struct drm_connector_helper_funcs .get_modes() hook (which is also not
supposed to return negative codes), or add it to other counts leading to
bogus values.

On the other hand, many of the struct drm_panel_funcs .get_modes() hooks
do return negative error codes, so handle them gracefully instead of
propagating further.

Return 0 for no modes, whatever the reason.

Cc: Neil Armstrong <neil.armstrong@linaro.org>
Cc: Jessica Zhang <quic_jesszhan@quicinc.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: stable@vger.kernel.org
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/79f559b72d8c493940417304e222a4b04dfa19c4.1709913674.git.jani.nikula@intel.comSigned-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/drm_panel.c

@@ -274,19 +274,24 @@ EXPORT_SYMBOL(drm_panel_disable);
  * The modes probed from the panel are automatically added to the connector
  * that the panel is attached to.
  *
- * Return: The number of modes available from the panel on success or a
- * negative error code on failure.
+ * Return: The number of modes available from the panel on success, or 0 on
+ * failure (no modes).
  */
 int drm_panel_get_modes(struct drm_panel *panel,
 			struct drm_connector *connector)
 {
 	if (!panel)
-		return -EINVAL;
+		return 0;
 
-	if (panel->funcs && panel->funcs->get_modes)
-		return panel->funcs->get_modes(panel, connector);
+	if (panel->funcs && panel->funcs->get_modes) {
+		int num;
 
-	return -EOPNOTSUPP;
+		num = panel->funcs->get_modes(panel, connector);
+		if (num > 0)
+			return num;
+	}
+
+	return 0;
 }
 EXPORT_SYMBOL(drm_panel_get_modes);