drm/mediatek: Add return value check when reading DPCD

Check the return value of drm_dp_dpcd_readb() to confirm that
AUX communication is successful. To simplify the code, replace
drm_dp_dpcd_readb() and DP_GET_SINK_COUNT() with drm_dp_read_sink_count().

Fixes: f70ac09 ("drm/mediatek: Add MT8195 Embedded DisplayPort driver")
Signed-off-by: Liankun Yang <liankun.yang@mediatek.com>
Reviewed-by: Guillaume Ranquet <granquet@baylibre.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20241218113448.2992-1-liankun.yang@mediatek.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

Author: yangliankun1

drivers/gpu/drm/mediatek/mtk_dp.c

@@ -2103,7 +2103,6 @@ static enum drm_connector_status mtk_dp_bdg_detect(struct drm_bridge *bridge)
 	struct mtk_dp *mtk_dp = mtk_dp_from_bridge(bridge);
 	enum drm_connector_status ret = connector_status_disconnected;
 	bool enabled = mtk_dp->enabled;
-	u8 sink_count = 0;
 
 	if (!mtk_dp->train_info.cable_plugged_in)
 		return ret;
@@ -2118,8 +2117,8 @@ static enum drm_connector_status mtk_dp_bdg_detect(struct drm_bridge *bridge)
 	 * function, we just need to check the HPD connection to check
 	 * whether we connect to a sink device.
 	 */
-	drm_dp_dpcd_readb(&mtk_dp->aux, DP_SINK_COUNT, &sink_count);
-	if (DP_GET_SINK_COUNT(sink_count))
+
+	if (drm_dp_read_sink_count(&mtk_dp->aux) > 0)
 		ret = connector_status_connected;
 
 	if (!enabled)