Of course you can use just one 12V PSU, and "split" or "forked" out a 5V to Rpi, using a popular LM2596 based switching power supply.  But it is always a good idea to "decouple" MCU/SBC/logic power from Motor/Relay/Solenoid power which as 
EMI/Back EMF/Noise Problems.  Usually mains/actuator voltage spikes and noises will escape through ground wires, so it is a good idea to decouple power/ground wires as much as possible.  So let the (mains) grounding at the 110AC power bar is the best you can do.  There are other tricks to avoid noise etc, eg. Using optical isolation at input side.  But then I am making my short answers too long! PS - For separate grounding circuits usually the newbies and even ninjas forget to short/connect separate logical grounds together.  Usually I deliberately do not warn/remind them before hand, and let them complete every wiring bit and find that nothing happens. I still would not tell them what goes wrong, and suggest them to google to find out the important  thing of common.  This way the newbies will remember this deep in their heart.  Cheers.