drm/tegra: dc: Silence RGB output deferred-probe error

Driver fails to probe with -EPROBE_DEFER if display output isn't ready
yet. This produces a bit noisy error message in KMSG during kernel's boot
up on Tegra20 and Tegra30 because RGB output tends to be probed earlier
than a corresponding voltage regulator driver.
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/gpu/drm/tegra/dc.c

@@ -2571,7 +2571,13 @@ static int tegra_dc_probe(struct platform_device *pdev)
 
 	err = tegra_dc_rgb_probe(dc);
 	if (err < 0 && err != -ENODEV) {
-		dev_err(&pdev->dev, "failed to probe RGB output: %d\n", err);
+		const char *level = KERN_ERR;
+
+		if (err == -EPROBE_DEFER)
+			level = KERN_DEBUG;
+
+		dev_printk(level, dc->dev, "failed to probe RGB output: %d\n",
+			   err);
 		return err;
 	}