msm: kgsl: Enable GMU processor clock gating

When the GMU enters the wait-for-interrupt state (WFI), the processor
clock should be gated to save power. Write the GMU_WFI_CONFIG register
to do this when we set up hardware clock gating.

Change-Id: Id97406a3e5ab9bb8a38c4a9289b9d99ce61ce2ab
Signed-off-by: Raghu Ananya Arabolu <rarabolu@codeaurora.org>