msm: kgsl: Add gmu enable check before GMU_AO_SPARE_CNTL

Few A6x GPUs are not having GMU, accessing GMU_AO_SPARE_CNTL
register on no GMU based GPUs can lead to out of bound access.
So ensure gmu enabled check present before GMU_AO_SPARE_CNTL.

Change-Id: Id26cbbbc24d2fdfc996aa0a20ac2f8a59e07d00d
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>