drm/bridge: tc358767: Do not cache dsi_lanes twice

The DSI lane count can be accessed via the dsi device pointer,
make use of that. No functional change.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Maxime Ripard <maxime@cerno.tech>
Cc: Robert Foss <robert.foss@linaro.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220624181902.151959-1-marex@denx.de