msm: kgsl: Do not expose gpurev in kgsl to the userspace

Zero out the the gpu_id/gpurev passed to the userpace in the
adreno_getproperty ioctl so they cannot use it in any way. The
userspace should only need the chipid for chip detection.

Change-Id: I1689c3cc7b31d5fb69b912f8551943e59b35ad07
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>