No, LVDS and emulated LVDS outputs in Cyclone® series devices cannot be tri-stated.
The only differential standard which is supported for outputs tri-state is Bus LVDS which is supported for Cyclone III and Cyclone IV devices.
If you need to tri-state the differential signals in Cyclone and Cyclone II devices, you can use two single ended 2.5V outputs, one inverted from the other. You will need an external resistor network to attenuate the single ended 2.5V signals to LVDS levels. Use IBIS simulation to find the best drive strength and resistor network values required to give you the LVDS levels required for your interface.