drm/amd/display: Add Audio readback registers

[Why]
Can be useful for verifying the correctness of audio output.

Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Ilya Bakoulin <Ilya.Bakoulin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>