PM: devfreq: Fix simple_ondemand crashing on startup

simple_ondemands private data must be set to NULL, otherwise we would
run into a NULL pointer in kgsl_devfreq_get_dev_status().

Change-Id: I6cc6a8b11e3b58b8c3e3c26d43ee36949cf62351
PM: devfreq: Always reflect a change of polling interval

Change-Id: Ie895bdf1ed1126e05483890f7ed64ac05890710a
PM: devfreq: Use high priority workqueue

It does not make sense to run kgsl on high and devfreq on regular
priority.

Change-Id: Ie5e6c9353a4e1324a6a49278e5ad3638462f551c