msm: kgsl: Don't let thermal control use invalid GPU frequencies

The lowest non-zero GPU frequency is only meant to be set when
the device enters SLEEP and the clocks are off.  Do not permit it
to be used as a thermal level.

Change-Id: I6eedf39849ec5211977f824148883e3694cae50b
Signed-off-by: Lucille Sylvester <lsylvest@codeaurora.org>