cpufreq: governor: Simplify performance and powersave governors

The performance and powersave cpufreq governors handle the
CPUFREQ_GOV_START event in the same way as CPUFREQ_GOV_LIMITS.
However, the cpufreq core always invokes cpufreq_governor() with the
event argument equal to CPUFREQ_GOV_LIMITS right after invoking it with
event equal to CPUFREQ_GOV_START.  As a result, for both the governors
in question, __cpufreq_driver_target() is executed twice in a row
with the same arguments which is not useful.

For this reason, simplify the performance and powersave governors
to handle the CPUFREQ_GOV_LIMITS event only as that's going to be
sufficient for the governor start too.

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