drm/amd/pm: Add GFXOFF interface for Vangogh

Steps to enter gfxoff mode on Vangogh

1. Send EnableGfxOff message to SMU
2. Send AllowGfxOff message to SMU

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