drm/amd/pm: integrate plpd allow/disallow into select_xgmi_plpd_policy in ppt level

The allow_xgmi_power_down(true/false) will be generally replaced by:
  - allow: select_xgmi_plpd_policy(XGMI_PLPD_DEFAULT)
  - disallow: select_xgmi_plpd_policy(XGMI_PLPD_DISALLOW)

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>