drm/msm: fix OF child-node lookup

Use the new of_get_compatible_child() helper to lookup the legacy
pwrlevels child node instead of using of_find_compatible_node(), which
searches the entire tree from a given start node and thus can return an
unrelated (i.e.  non-child) node.

This also addresses a potential use-after-free (e.g. after probe
deferral) as the tree-wide helper drops a reference to its first
argument (i.e. the probed device's node).

While at it, also fix the related child-node reference leak.

Fixes: e2af8b6b0ca1 ("drm/msm: gpu: Use OPP tables if we can")
Cc: stable <stable@vger.kernel.org>     # 4.12
Cc: Jordan Crouse <jcrouse@codeaurora.org>
Cc: Rob Clark <robdclark@gmail.com>
Cc: David Airlie <airlied@linux.ie>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>

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

@@ -633,8 +633,7 @@ static int adreno_get_legacy_pwrlevels(struct device *dev)
 	struct device_node *child, *node;
 	int ret;
 
-	node = of_find_compatible_node(dev->of_node, NULL,
-		"qcom,gpu-pwrlevels");
+	node = of_get_compatible_child(dev->of_node, "qcom,gpu-pwrlevels");
 	if (!node) {
 		dev_err(dev, "Could not find the GPU powerlevels\n");
 		return -ENXIO;
@@ -655,6 +654,8 @@ static int adreno_get_legacy_pwrlevels(struct device *dev)
 			dev_pm_opp_add(dev, val, 0);
 	}
 
+	of_node_put(node);
+
 	return 0;
 }