drm/amdgpu/powerplay: split out common smu7 BACO code

Several of the BACO functions are common across smu7-based
asics.  Split the common code out.

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>