msm: kgsl: Lower bounds check the number of power levels
A misconfigured board file could cause an out-of-bounds
array access through this value.

Change-Id: I64bbad0c096a2efe65376991537d810141370d72
CRs-Fixed: 439230
Signed-off-by: Lucille Sylvester <lsylvest@codeaurora.org>