cpufreq: governor: Symmetrize cpu_dbs_info initialization and cleanup

Make the initialization of struct cpu_dbs_info objects in
alloc_policy_dbs_info() and the code that cleans them up in
free_policy_dbs_info() more symmetrical.  In particular,
set/clear the update_util.func field in those functions along
with the policy_dbs field.

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