drm/radeon: don't allow RADEON_GEM_DOMAIN_CPU for command submission

commit ec65da385d46f63740c1c9230b891a6dcbd64c71 upstream.

It hangs the hardware.

Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>