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.

TODO: sysfs tuning of polling interval and thresholds

Change-Id: I52be6c1c683f0e9825f9964be69ed1b11d6451a7
Signed-off-by: Andrew Dodd <atd7@cornell.edu>