drm/amd/pm: move those code piece used by Stoney only to smu8_hwmgr.c

Instead of putting them in amdgpu_dpm.c.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>