drm/bridge: tc358767: Fix DP bridge mode detection from DT endpoints

drm/bridge: tc358767: Fix DP bridge mode detection from DT endpoints

Per toshiba,tc358767.yaml DT binding document, port@2 the output (e)DP
port is optional. In case this port is not described in DT, the bridge
driver operates in DPI-to-DP mode. Make sure the driver treats this as
a valid mode of operation instead of reporting invalid mode.

Fixes: d6341964b973 ("drm/bridge: tc358767: Detect bridge mode from connected endpoints in DT")
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Jonas Karlman <jonas@kwiboo.se>
Cc: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Maxime Ripard <maxime@cerno.tech>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Robert Foss <robert.foss@linaro.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220429204625.241591-1-marex@denx.de