drm/mediatek: mtk-dpi: Set out_fmt from config if not the last bridge

atomic_get_output_bus_fmts() is only called when the bridge is the last
element in the bridge chain.

If mtk-dpi is not the last bridge, the format of output_bus_cfg is
MEDIA_BUS_FMT_FIXED, and mtk_dpi_dual_edge() will fail to write correct
value to regs.

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

drivers/gpu/drm/mediatek/mtk_dpi.c

@@ -610,6 +610,10 @@ static int mtk_dpi_bridge_atomic_check(struct drm_bridge *bridge,
 
 	out_bus_format = bridge_state->output_bus_cfg.format;
 
+	if (out_bus_format == MEDIA_BUS_FMT_FIXED)
+		if (dpi->conf->num_output_fmts)
+			out_bus_format = dpi->conf->output_fmts[0];
+
 	dev_dbg(dpi->dev, "input format 0x%04x, output format 0x%04x\n",
 		bridge_state->input_bus_cfg.format,
 		bridge_state->output_bus_cfg.format);