drm/amdgpu/pm: add gfx_off_control for yellow carp

This patch implements gfx_off_control.

Signed-off-by: Aaron Liu <aaron.liu@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>