drm/amdkfd: CRIU fix a NULL vs IS_ERR() check

The kfd_process_device_data_by_id() does not return error pointers,
it returns NULL.

Fixes: bef153b7 ("drm/amdkfd: CRIU implement gpu_id remapping")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: David Yat Sin <david.yatsin@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>