drm/amdgpu: Enable SDMA MGCG for Vangogh

Add flags AMD_CG_SUPPORT_SDMA_MGCG for Vangogh.
Start to open sdma mgcg from firmware version 70.

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