cpufreq: governors: Get rid of dbs_data->enable field

CPUFREQ_GOV_START/STOP are called only once for all policy->cpus and hence we
don't need to adapt cpufreq_governor_dbs() routine for multiple calls.

So, this patch removes dbs_data->enable field entirely. And rearrange code a
bit.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>