PM: domains: Clean up some code in pm_genpd_init() and genpd_remove()

To improve the readability of the code, let's move the parts that deals
with allocation/freeing of data, into two separate functions.

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