drm/amd/pm: init plpd_mode properly for different asics

Assign DEFAULT mode if it supports plpd, otherwise keeps NONE

v2: reduce ip version checks

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>