msm: kgsl: Zero the adreno ioctl command buffer

The kernel command buffer is not zeroed in the adreno ioctls,
and may contain garbage. The garbage value can lead to
unexpected results.

CRs-Fixed: 993518
Change-Id: I75033cdf4637881ecd6fa4dd31aea083b134e6d2
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>