drm/amdgpu: Add NPS2 to DPX compatible mode

Compute partition DPX is possible in NPS2 mode. Update the compatible
modes for DPX.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>