->related_cpus is empty at this point of time and copying ->cpus to it
or orring ->related_cpus with ->cpus would result in the same value. But
cpumask_copy makes it rather clear.

Reviewed-by: Saravana Kannan <skannan@codeaurora.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>