drm/amdgpu: implement new cgs interface for acpi function

Add a new driver internal interface for accessing ACPI
methods.  These will be used by various new components
including powerplay.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>