drm/radeon: don't allow audio on DCE6

commit 1cbcca302a318499f20a512847c5d6a510c08c35 upstream.

It's not supported yet.  Fixes display issues when
users force it on.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>