drm/amd/pm: deprecate allow_xgmi_power_down interface

Replace with set_plpd_mode uniformly for places to use.

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>