drm/amdgpu: Define virtual display ip blocks.

For virtual display feature, define virtual display ip blocks, and set
dce_virtual_ip_funcs to DCE block.

Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>