drm/amdgpu/powerplay: add core support for pre-SOC15 baco

This adds core support for BACO on pre-vega asics.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>