drm/amdgpu: add vangogh apu flag

This patch is to add vangogh apu flag to support more kickers that
belongs vangogh series.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>