msm: kgsl: Add gpu_frequency trace during SLUMBER entry and exit

Add gpu_frequency trace during SLUMBER entry and exit. This is
needed to track GPU frequency across idle and active state
transitions.

Change-Id: Ib782805a1776e4c0fb879fc34dc2e1cd11cf6791
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>