PM / Domains: Convert to dev_to_genpd_safe() in genpd_syscore_switch()

The intent with walking the gpd_list via calling genpd_present() from
genpd_syscore_switch(), is to make sure the dev->pm_domain pointer belongs
to a registered genpd. However, as a genpd can't be removed if there is a
device attached to it, let's convert to use the quicker dev_to_genpd_safe()
instead.

Due to the above change, this allows us to clean up genpd_present() and
move it inside CONFIG_PM_GENERIC_DOMAINS_OF, so let's do that as well.

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