[how]
Add support for the virtual dal flag. This flag should skip
most/all of DC construction since the HW isn't accessible, but
still construct WindowsDM (almost) normally but with only SW display
targets
Signed-off-by: Jun Lei <Jun.Lei@amd.com> Reviewed-by: Aric Cyr <Aric.Cyr@amd.com> Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com> Signed-off-by: Alex Deucher <alexander.deucher@amd.com>