kgsl: Implement conservative scaling policy

This behaves similarly to the conservative cpufreq governor,
which is similar to how most Mali GPU scaling algorithms behave.

The "trustzone" policy that is default relies on an undocumented
black box to make scaling decisions.

How to enable:
echo conservative > /sys/devices/platform/kgsl-3d0.0/kgsl/kgsl-3d0/pwrscale/policy

Conservative attr group:
/sys/devices/platform/kgsl-3d0.0/kgsl/kgsl-3d0/pwrscale/conservative

Polling interval (10-1000):
/sys/devices/platform/kgsl-3d0.0/kgsl/kgsl-3d0/pwrscale/conservative/polling_interval

Enable stats printing (causes microlags on low polling intervals):
echo 1 > /sys/devices/platform/kgsl-3d0.0/kgsl/kgsl-3d0/pwrscale/conservative/print_stats

NOTE:
- perms are set to 666 for testing purposes

Change-Id: I9d854a73981515fc9850104302804928b04c4687
Signed-off-by: Andrew Dodd <atd7@cornell.edu>