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