drm/amd/display: Disable virtual links

[Why]
This could work around the issue with DP tunneling producing
a NULL pointer dereference.

[How]
Disable the virtual links.

Reviewed-by: Tom Chung <chiahsuan.chung@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Stylon Wang <stylon.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>