drm/amd/display: disable lttpr for Navi

Signed-off-by: Hugo Hu <hugo.hu@amd.com>
Reviewed-by: Abdoulaye Berthe <Abdoulaye.Berthe@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>