drm/amdgpu: improve error handling in pcie_bw

1. Initialize the counters to 0 in case the callback
   fails to initialize them.
2. The counters don't exist on APUs so return an error
   for them.
3. Return an error if the callback doesn't exist.
Reviewed-by: Yong Zhao <Yong.Zhao@amd.com>
Reviewed-By: Kent Russell <kent.russell@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_pm.c

@@ -1646,9 +1646,15 @@ static ssize_t amdgpu_get_pcie_bw(struct device *dev,
 {
 	struct drm_device *ddev = dev_get_drvdata(dev);
 	struct amdgpu_device *adev = ddev->dev_private;
-	uint64_t count0, count1;
+	uint64_t count0 = 0, count1 = 0;
 	int ret;
 
+	if (adev->flags & AMD_IS_APU)
+		return -ENODATA;
+
+	if (!adev->asic_funcs->get_pcie_usage)
+		return -ENODATA;
+
 	ret = pm_runtime_get_sync(ddev->dev);
 	if (ret < 0)
 		return ret;