drm/amd/display: use preferred link settings for dp signal only

[ Upstream commit abf34ca465f5cd182b07701d3f3d369c0fc04723 ]

[why]
We set preferred link settings for virtual signal. However we don't support
virtual signal for UHBR link rate. If preferred is set to UHBR link rate, we
will allow virtual signal with UHBR link rate which causes system crashes.

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Acked-by: Zaeem Mohamed <zaeem.mohamed@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>
Signed-off-by: Sasha Levin <sashal@kernel.org>