drm/amd/pm: Add xgmi plpd policy to pm_policy

Add support to set XGMI PLPD policy levels through 'pm_policy/xgmi_plpd'
sysfs node.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Asad Kamal <asad.kamal@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>