cpufreq.chart.py: support dynamically falling back from accurate to inaccurate mode

cpufreq.chart.py: support dynamically falling back from accurate to inaccurate mode

If we switch the 'schedutil' cpufreq governor on, the 'time_in_state'
data becomes useless. And since cpufreq governors can be switched at any
time, we might as well make it possible to support transitioning between
governors without any bogus data appearing in our charts.

Before this commit, switching to 'schedutil' results in zeroed values,
and switching back to any other governor causes a huge spike datapoint
in the timeline.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>