drm/radeon/audio: don't hardcode CRTC id

commit 0aecb5a4ba1ea4167f31d1790eec027f1e658f2d upstream.

This is based on info released by AMD, should allow using audio in much
more cases.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>