drm/amdgpu: Hardcode virtual DCE vblank / scanout position return values

For virtual display feature, by hardcoding 0 for the vblank counter and
-EINVAL for the scanout position return value, we signal to the core DRM code that there are
no hardware counters we can use for these.

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