drm/amd/display: Avoid virtual stream encoder if not explicitly requested

Virtual stream encoder should not be a free match for thunderbolt or
usbc, and thus should be avoided.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>