PM: devfreq: Use high priority workqueue

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

Change-Id: Ie5e6c9353a4e1324a6a49278e5ad3638462f551c
msm: mdss: Use interruptible threads

Based on @cyanogen's findings, use interruptible threads to reduce
cpu load.

Change-Id: Ice983fd565d5defaa4a09e35d228bfd39f84c54e