PM / devfreq: bw_hwmon: Take at least one sample per decision window

Due to the way the short samples are triggered, when the traffic is very
low, we could have a decision window where no sample is taken. Fix this by
forcing a sample at the end of every decision window as long as that is the
first sample for that decision window or the sample window won't be smaller
than the sample_ms tunable.

Change-Id: Ia33b59fbff9ff4058c06e24208d1e037d6e9eda9
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>