We accidentally return ERR_PTR(0) which is NULL.  The caller is not
expecting that and it leads to an Oops.

Fixes: dd59239a ("amdkfd: init aperture once per process")
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: NFelix Kuehling <felix.kuehling@amd.com>
Signed-off-by: NOded Gabbay <oded.gabbay@gmail.com>