PM / Domains: Silence compiler warning for an unused function

The only remaining caller of genpd_poweron() is conditionally compiled
based on CONFIG_PM_GENERIC_DOMAINS_OF, so we get a warning when that is
unset.

By moving the locking/unlocking of the genpd outside genpd_poweron(), thus
to the caller, genpd_poweron() becomes redundant.

Within this context let's then rename the wrapper function,
__genpd_poweron(), to genpd_poweron() as it will then be consistent with
its friend genpd_poweroff().

This change silence the warning about the unused function.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Fixes: ea823c7c

 "PM / Domains: Remove pm_genpd_poweron() API"
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: 4.4+ <stable@vger.kernel.org> # 4.4+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>