drm/amd/display: support virtual DCN

[why]
DAL3 should support SRIOV

[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>