drm/amdgpu: add Vangogh APU flag to IP discovery path

Add the missing apu flag for Vangogh when using IP discovery code path
to initialize IPs

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