msm: kgsl: Add functions for enabling / disabling a powerscale policy

Sometimes you want to enable or disable a policy for a bit without
actually swapping out the policy.  These functions allow the kernel
to control the policy functionality.

Change-Id: Ic0dedbad4406e5fffe1ebd8e8ba4e6bf84d82203
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>