drm/i915: Initiate DP link training only on the lanes we'll be using

Limit the link training setting command to the lanes needed for the
current mode. It seems vaguely possible that a monitor will try to
train the other lanes and fail in some way, so this seems like the
safer plan.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>

drivers/gpu/drm/i915/intel_dp.c

@@ -1564,8 +1564,9 @@ intel_dp_set_link_train(struct intel_dp *intel_dp,
 
 	ret = intel_dp_aux_native_write(intel_dp,
 					DP_TRAINING_LANE0_SET,
-					intel_dp->train_set, 4);
-	if (ret != 4)
+					intel_dp->train_set,
+					intel_dp->lane_count);
+	if (ret != intel_dp->lane_count)
 		return false;
 
 	return true;