drm/amdgpu: Initialize dce_virtual_display_funcs.

For virtual display feature, initialize dce_virtual_display_funcs,
which will be used in function dce_virtual_set_display_funcs.

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>