drm/amdgpu: use kobj_to_dev()

BugLink: http://bugs.launchpad.net/bugs/1546572

Use kobj_to_dev() instead of open-coding it.
Signed-off-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit cc29ec87)
Signed-off-by: Alberto Milone <alberto.milone@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

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

@@ -326,7 +326,7 @@ static struct attribute *hwmon_attributes[] = {
 static umode_t hwmon_attributes_visible(struct kobject *kobj,
 					struct attribute *attr, int index)
 {
-	struct device *dev = container_of(kobj, struct device, kobj);
+	struct device *dev = kobj_to_dev(kobj);
 	struct amdgpu_device *adev = dev_get_drvdata(dev);
 	umode_t effective_mode = attr->mode;