drm/amd/display: fix DP audio settings

[why]
Audio channel layout for 5.1ch is not correct

[how]
Add the audio layout for 5.1ch (channel_count = 6).
Add divided by zero check.

Reviewed-by: Zhan Liu <zhan.liu@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>