PM: devfreq: Use high priority workqueue

It does not make sense to run kgsl on high and devfreq on regular
priority.
defconfig update
PM: devfreq: Define tunables with a static scope
PM: devfreq: Fix derpage

Forgot to git add. Note to self: Don't push when you're
fucking pissed about fucking useless discussions.
PM: devfreq: Check for GPU busy time before sending the sample

Sometimes GPU busy time is very low during sampling. Accumulate
the statistics and send them with the next sample.

Change-Id: I77e568b48afc3a4922c7c341d9478705b7ae3a30
Signed-off-by: Suman Tatiraju <sumant@codeaurora.org>