Enable gfxoff in driver side based on SMC#73.3

v2: fix typo 'Eanble' --> 'Enable'
Signed-off-by: Chengming Gui <Jack.Gui@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>