drm/amdgpu: memset gds_info struct in info ioctl

Avoids possibility that info may leak via the uninitialized
_pad element.

Noticed-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>