PM / Domains: Provide a dummy dev_gpd_data() when generic domains are not used

dev_gpd_data() is a generic macro, also useful for drivers. Hence it should
be available also when CONFIG_PM_GENERIC_DOMAINS is not selected. OTOH,
to_gpd_data() is so far unused outside of the generic PM domain code and
does not seem to be very useful without CONFIG_PM_GENERIC_DOMAINS.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

include/linux/pm_domain.h

@@ -101,12 +101,12 @@ struct generic_pm_domain_data {
 	bool need_restore;
 };
 
+#ifdef CONFIG_PM_GENERIC_DOMAINS
 static inline struct generic_pm_domain_data *to_gpd_data(struct pm_domain_data *pdd)
 {
 	return container_of(pdd, struct generic_pm_domain_data, base);
 }
 
-#ifdef CONFIG_PM_GENERIC_DOMAINS
 static inline struct generic_pm_domain_data *dev_gpd_data(struct device *dev)
 {
 	return to_gpd_data(dev->power.subsys_data->domain_data);
@@ -207,6 +207,10 @@ static inline bool default_stop_ok(struct device *dev)
 	return false;
 }
 #define pm_domain_always_on_gov NULL
+static inline struct generic_pm_domain_data *dev_gpd_data(struct device *dev)
+{
+	return NULL;
+}
 #endif
 
 static inline int pm_genpd_remove_callbacks(struct device *dev)