drm/amd/display: skip audio config for virtual signal

[WHY & HOW]
IGT's kms_hdmi_inject (subtest inject_audio) triggers a warning message
from dce_aud_az_configure. This can be fixed by checking Virtual signal
that does not need to configure AZ audio since it does not have any.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>