drm/amdkfd: NULL dereference involving create_process()

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: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>