cpufreq: schedutil: Use the frequency below the target if they're close

Schedutil targets a frequency tipping point of 80% to vote for a higher
frequency when utilization crosses that threshold.

Since the tipping point calculation is done without regard to the size of
the gap between each frequency step, this often results in a large
frequency jump when it isn't strictly necessary, which hurts energy
efficiency.

For example, if a CPU has 2000 MHz and 3000 MHz frequency steps, and
schedutil targets a frequency of 2005 MHz, then the 3000 MHz frequency step
will be used even though the target frequency of 2005 MHz is very close to
2000 MHz. In this hypothetical scenario, using 2000 MHz would clearly
satisfy the system's performance needs while consuming less energy than the
3000 MHz step.

To counter-balance the frequency tipping point, add a frequency tipping
point in the opposite direction to prefer the frequency step below the
calculated target frequency when the target frequency is less than 20%
higher than that lower step. A threshold of 20% was empirically determined
to provide significant energy savings without really impacting performance.

This improves schedutil's energy efficiency on CPUs which have large gaps
between their frequency steps, as is often the case on ARM.

Change-Id: Ie75b79e5eb9f52c966848a9fb1c8016d7ae22098
Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>