drm/mediatek: dpi: Fix NULL dereference in mtk_dpi_bridge_atomic_check

bridge->driver_private is not set (NULL) so use bridge_to_dpi(bridge)
like it's done in bridge_atomic_get_output_bus_fmts

Fixes: ec8747c5 ("drm/mediatek: dpi: Add bus format negotiation")
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Tested-by: Hsin-Yi Wang <hsinyi@chromium.org>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

drivers/gpu/drm/mediatek/mtk_dpi.c

@@ -605,7 +605,7 @@ static int mtk_dpi_bridge_atomic_check(struct drm_bridge *bridge,
 				       struct drm_crtc_state *crtc_state,
 				       struct drm_connector_state *conn_state)
 {
-	struct mtk_dpi *dpi = bridge->driver_private;
+	struct mtk_dpi *dpi = bridge_to_dpi(bridge);
 	unsigned int out_bus_format;
 
 	out_bus_format = bridge_state->output_bus_cfg.format;