PM / Domains: Handle safely genpd_syscore_switch() call on non-genpd device

genpd_syscore_switch() had two problems:
 1. It silently assumed that device, it is being called for, belongs
    to generic power domain and used container_of() on its power
    domain pointer.  Such assumption might not be true always.

2. It iterated over list of generic power domains without holding
   gpd_list_lock mutex thus list could have been modified at the same
   time.

Usage of genpd_lookup_dev() solves both problems as it is safe a call
for non-generic power domains and uses mutex when iterating.
Reported-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/base/power/domain.c

@@ -1148,8 +1148,8 @@ static void genpd_syscore_switch(struct device *dev, bool suspend)
 {
 	struct generic_pm_domain *genpd;
 
-	genpd = dev_to_genpd(dev);
-	if (!pm_genpd_present(genpd))
+	genpd = genpd_lookup_dev(dev);
+	if (!genpd)
 		return;
 
 	if (suspend) {