msm: kgsl: Change pwrlevels array index type to unsigned int

Change the array index 'i' of pwrlevels[] to unsigned int, which can't be
decremented beyond zero.

Change-Id: I9b646fccf03437f6facf0d5938f097916d13d2f2
Signed-off-by: Venkateswara Rao Tadikonda <vtadik@codeaurora.org>