msm: kgsl: Modify gpu_max_clock sysfs node

The intent of the gpu_max_clock node is to set / show the maximum
GPU frequency. To accomplish this, set the max_pwrlevel instead of
thermal_pwrlevel when the gpu_max_clock node is set.

Change-Id: I90c5e638f22b363c60d351c8c78814df67b256a2
Signed-off-by: Raghu Ananya Arabolu <rarabolu@codeaurora.org>