cpufreq: governors: Fix long idle detection logic in load calculation

According to current code implementation, detecting the long
idle period is done by checking if the interval between two
adjacent utilization update handlers is long enough. Although
this mechanism can detect if the idle period is long enough
(no utilization hooks invoked during idle period), it might
not cover a corner case: if the task has occupied the CPU
for too long which causes no context switches during that
period, then no utilization handler will be launched until this
high prio task is scheduled out. As a result, the idle_periods
field might be calculated incorrectly because it regards the
100% load as 0% and makes the conservative governor who uses
this field confusing.

Change the detection to compare the idle_time with sampling_rate
directly.

Reported-by: Artem S. Tashkinov <t.artem@mailcity.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Cc: All applicable <stable@vger.kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>