msm: kgsl: Avoid accessing GMU register if GMU is not enabled

Add a check to make sure GMU is enabled before accessing GMU
register in a6xx_hwcg_set to avoid "Out of bounds" register
access warnings.

Change-Id: I735596676191dc9683423e605b780d89248f8ccb
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>