msm: kgsl: Don't spam the kernel log if qcom,step-pwrlevel isn't found

qcom,step-pwrlevel is a specific parameter that only applies to
certain targets.  Don't spam the world if it isn't specified.

CRs-fixed: 556418
Change-Id: Ic0dedbad65dbdcf9bc227fef4a796e22131ba68f
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>