[why]
1. As recommended by hardware team, don't enable APG when stream is not
   enabled.
2. Move audio stream encoder programming into link_hwss.

[how]
1. Merge dp_audio_enable into enable audio stream hw sequence.
2. Move stream encoder programming into link hwss level to unify stream
   encoder programming interface.
Tested-by: NMark Broadworth <mark.broadworth@amd.com>
Reviewed-by: NCharlene Liu <Charlene.Liu@amd.com>
Acked-by: NRodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: NWenjing Liu <wenjing.liu@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>