drm/amdgpu: Add gfx_off support in smu through pp_set_powergating_by_smu

we can take gfx off feature as gfx power gate. gfx off feature is also
controled by smu. so add gfx_off support in pp_set_powergating_by_smu.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>