drm/amdgpu/dce_virtual: add Arcturus virtual display support

Virtual dce is a sw only display driver for emulation and
virtualization and cases where we want to use a virtual
display subsystem.

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>