drm/amd/display: Revert "drm/amd/display: allow edp updates for virtual signal"

This reverts commit 4ad3ee5c.

[WHY & HOW]
Virtual signal is not supported as audio capable by DC.

Reviewed-by: Chao-kai Wang <stylon.wang@amd.com>
Acked-by: Roman Li <roman.li@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>