drm/amd/display: Don't enable AFMT for DP audio stream

[WHY]
AFMT is unused for DP audio, so powering it on for DP is unnecessary.

[HOW]
APG block should be powered down instead, however HW defaults to shutdown
state when not enabled so no further work is required.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>