PM: devfreq: Use high priority workqueue

It does not make sense to run kgsl on high and devfreq on regular
priority.
PM / devfreq: Fix simple_ondemand crashing
PM: devfreq: Always reflect a change of polling interval
PM: devfreq: Fix show available freqs
PM: devfreq: Add conservative governor

A GPU governor is not rocket science. This one gets the job done pretty
smoothly while complying with devfreq framework rules.

Change-Id: I64779aca05978834c791dad448283a7496244db5
PM: devfreq: Add some conservativeness
PM: devfreq: conservative: Return 0 if GPU just started

Bailing out early is perfectly fine, so do not trigger an error in
update_devfreq.
PM: devfreq: Add sysfs nodes for devfreq governors

Change-Id: Ia4c5836a4e5095d499ca79d7e1ead61ddcfb0215
PM: devfreq: Move governor node to gpufreq

Final path is: /sys/class/devfreq/fdb00000.qcom,kgsl-3d0/gpufreq