drm/amdgpu: Initialize dce_virtual_ip_funcs

For virtual display feature, first need to initialize dce_virtual_ip_funcs,
which will be used when set ip blocks.

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>