drm/msm/adreno: Do not print error on "qcom, gpu-pwrlevels" absence

Booting the adreno driver on a imx53 board leads to the following
error message:

adreno 30000000.gpu: [drm:adreno_gpu_init] *ERROR* Could not find the GPU powerlevels

As the "qcom,gpu-pwrlevels" property is optional and never present on
i.MX5, turn the message into debug level instead.
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Jeffrey Hugo <jeffrey.l.hugo@gmail.com>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>

drivers/gpu/drm/msm/adreno/adreno_gpu.c

@@ -826,7 +826,7 @@ static int adreno_get_legacy_pwrlevels(struct device *dev)
 
 	node = of_get_compatible_child(dev->of_node, "qcom,gpu-pwrlevels");
 	if (!node) {
-		DRM_DEV_ERROR(dev, "Could not find the GPU powerlevels\n");
+		DRM_DEV_DEBUG(dev, "Could not find the GPU powerlevels\n");
 		return -ENXIO;
 	}