drm: rcar-du: Use dev_err_probe() to record cause of KMS init errors

The (large) rcar_du_modeset_init() function can fail for many reasons,
two of two involving probe deferral. Use dev_err_probe() in those code
paths to record the cause of the probe deferral, in order to help
debugging probe issues.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>

drivers/gpu/drm/renesas/rcar-du/rcar_du_drv.c

@@ -696,6 +696,10 @@ static int rcar_du_probe(struct platform_device *pdev)
 	/* DRM/KMS objects */
 	ret = rcar_du_modeset_init(rcdu);
 	if (ret < 0) {
+		/*
+		 * Don't use dev_err_probe(), as it would overwrite the probe
+		 * deferral reason recorded in rcar_du_modeset_init().
+		 */
 		if (ret != -EPROBE_DEFER)
 			dev_err(&pdev->dev,
 				"failed to initialize DRM/KMS (%d)\n", ret);

drivers/gpu/drm/renesas/rcar-du/rcar_du_kms.c

@@ -935,7 +935,8 @@ int rcar_du_modeset_init(struct rcar_du_device *rcdu)
 	/* Initialize the Color Management Modules. */
 	ret = rcar_du_cmm_init(rcdu);
 	if (ret)
-		return ret;
+		return dev_err_probe(rcdu->dev, ret,
+				     "failed to initialize CMM\n");
 
 	/* Create the CRTCs. */
 	for (swindex = 0, hwindex = 0; swindex < rcdu->num_crtcs; ++hwindex) {
@@ -955,7 +956,8 @@ int rcar_du_modeset_init(struct rcar_du_device *rcdu)
 	/* Initialize the encoders. */
 	ret = rcar_du_encoders_init(rcdu);
 	if (ret < 0)
-		return ret;
+		return dev_err_probe(rcdu->dev, ret,
+				     "failed to initialize encoders\n");
 
 	if (ret == 0) {
 		dev_err(rcdu->dev, "error: no encoder could be initialized\n");