drm/amdgpu: Add virtual connector and encoder macros.

For virtual display feature, add virtual connector and encoder macros.

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>