Export apis for enabling/disabling SMU gfxoff support.

v2: fit the latest gfxoff support framework
v3: add feature_mask control
Signed-off-by: NEvan Quan <evan.quan@amd.com>
Reviewed-by: Huang Rui <ray.huang at amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>