drm/radeon: fix DP audio support for APU with DCE4.1 display engine

commit fe6fc1f1 upstream.

Properly setup the DFS divider for DP audio for DCE4.1.

Signed-off-by: Slava Grigorev <slava.grigorev@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>