drm/amd/display: correct apg audio channel enable golden value

Hardware team has recommended to generically hard code this register to
0xFF as part of the effort to eventually remove this control.  However
we set it to 0xF instead.
This causes 4 of audio 8ch to be muted.

Reviewed-by: Ariel Bernstein <eric.bernstein@amd.com>
Acked-by: Agustin Gutierrez Sanchez <agustin.gutierrez@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>