drm/amd/display: support spdif

[Description]
port spdif fix to staging:
 spdif hardwired to afmt inst 1.
 spdif func pointer
 spdif resource allocation (reserve last audio endpoint for spdif only)

Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>