vmwgfx: information leak in vmw_execbuf_copy_fence_user()

If ret is non-zero then we don't initialize the struct which leaks
stack information to user space.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Vasiliy Kulikov <segoon@openwall.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>