sched: cpufreq: Update cpufreq once in a WALT window

Multiple updates to cpufreq are unnecessary and potentially
buggy due to the dependence on current frequency. Ensure
that if WALT is enabled, we send only one load report per
window

Change-Id: I2a4e11e237a1cc6d2910a341c5ac20ea847befd8
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>