msm: kgsl: Enable frequency cycling for thermal mitigation

Allow thermal mitigation to select effective GPU frequencies
between the supported frequencies.  This gives more flexiblity
in the thermal performance limitations.

Change-Id: I618da263b02f09f293d737a13de567a3c31b37f0
Signed-off-by: Lucille Sylvester <lsylvest@codeaurora.org>