PM / Domains: Fix unsafe iteration over modified list of domains

of_genpd_remove_last() iterates over list of domains and removes
matching element thus it has to use safe version of list iteration.

Fixes: 17926551 (PM / Domains: Add support for removing nested PM domains by provider)
Cc: 4.9+ <stable@vger.kernel.org> # 4.9+
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>