drm/amd/display: refactor enable/disable audio stream hw sequence

[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: Mark Broadworth <mark.broadworth@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>