Revert "msm: kgsl: Do not memset pages to zero while adding to pool"

This reverts commit 90d6246f.

To address the launch latency issue seen because of increase in
memory allocation time.

Change-Id: I147ca8607337541b7a29056b4bd1b46aa374c6e3
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>