drm/amd/powerplay: refine powerplay code.

delete struct smumgr, put smu backend function table
in struct hwmgr

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