PM / Domains: Check for existing PM domain in dev_pm_domain_attach()

Instead of checking if an existing PM domain pointer has been assigned in
genpd_dev_pm_attach() and acpi_dev_pm_attach(), move the check to the
common path in dev_pm_domain_attach(), thus potentially avoid one
unnecessary check.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/device_pm.c

@@ -1259,9 +1259,6 @@ int acpi_dev_pm_attach(struct device *dev, bool power_on)
 	if (!adev)
 		return -ENODEV;
 
-	if (dev->pm_domain)
-		return -EEXIST;
-
 	/*
 	 * Only attach the power domain to the first device if the
 	 * companion is shared by multiple. This is to prevent doing power