cpufreq: governor: Narrow down the dbs_data_mutex coverage

Since cpufreq_governor_dbs() is now always called with policy->rwsem
held, it cannot be executed twice in parallel for the same policy.
Thus it is not necessary to hold dbs_data_mutex around the invocations
of cpufreq_governor_start/stop/limits() from it as those functions
never modify any data that can be shared between different policies.

However, cpufreq_governor_dbs() may be executed twice in parallal
for different policies using the same gov->gdbs_data object and
dbs_data_mutex is still necessary to protect that object against
concurrent updates.

For this reason, narrow down the dbs_data_mutex locking to
cpufreq_governor_init/exit() where it is needed and rename the
mutex to gov_dbs_data_mutex to reflect its purpose.

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