PM / Domains: Search for the CPU device outside the genpd lock

While attaching/detaching a device to a PM domain (genpd) with
GENPD_FLAG_CPU_DOMAIN set, genpd iterates the cpu_possible_mask to
check whether or not the device corresponds to a CPU. This iteration
is done while holding the genpd's lock, which is unnecessary.

Avoid the locking by restructuring the corresponding code a bit.

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