msm: kgsl: Fix gmu_dev_ops null pointer deference

Add a NULL check for gmu_dev_ops to fix NULL pointer
dereference on non gmu devices.

Change-Id: I01cad027557014e45727d88aba96a3d3d9367b0c
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>