msm: kgsl: Fixing an uninitialized variable in _gpuobj_map_useraddr()

Uninitalized kernel variable passes to userspace, it may have data
from stack. To avoid data leak from kernel to userspace initializes
struct kgsl_gpuobj_import_useraddr to zero.

CRs-Fixed: 2096006
Change-Id: Ib79b030cd5e3edd7279632af20dc3fac95eb73d4
Signed-off-by: Archana Obannagari <aobann@codeaurora.org>