drm/amd/powerplay: add mmhub powergating by smu in powerplay

new generic interface is added in powerplay.

Signed-off-by: Eric Huang <JinHuiEric.Huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>