msm: kgsl: Use signed integers for power level comparsions

The code that clamped the power levels to the requested minimum and
maximum values was mixing comparison signed and unsigned integers with
predictiably faulty results.  Move all values to signed integers to
handle negative power level numbers correctly

CRs-fixed: 427670
Change-Id: Ic0dedbada6153dc0b109923b376c3aa9a6abbeee
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>