drm/bridge: tc358767: fix opencoded use of drm_panel_*

Replace open coded version with call to drm_panel_get_modes().

Include change to deal with the possible negative
return values from drm_panel_get_modes()

v2:
- Added more info to changelog (Philipp)
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Cc: Andrzej Hajda <a.hajda@samsung.com>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
Cc: Jonas Karlman <jonas@kwiboo.se>
Cc: Jernej Skrabec <jernej.skrabec@siol.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190804201637.1240-2-sam@ravnborg.org

drivers/gpu/drm/bridge/tc358767.c

@@ -1312,7 +1312,7 @@ static int tc_connector_get_modes(struct drm_connector *connector)
 {
 	struct tc_data *tc = connector_to_tc(connector);
 	struct edid *edid;
-	unsigned int count;
+	int count;
 	int ret;
 
 	ret = tc_get_display_props(tc);
@@ -1321,11 +1321,9 @@ static int tc_connector_get_modes(struct drm_connector *connector)
 		return 0;
 	}
 
-	if (tc->panel && tc->panel->funcs && tc->panel->funcs->get_modes) {
-		count = tc->panel->funcs->get_modes(tc->panel);
-		if (count > 0)
-			return count;
-	}
+	count = drm_panel_get_modes(tc->panel);
+	if (count > 0)
+		return count;
 
 	edid = drm_get_edid(connector, &tc->aux.ddc);