If you assume the vi+ and vi- are such as to keep the circuit in the linear mode, then you know that, due to the current mirror, the current in the differential stage is twice the (I) current that is going to Mb1 of the mirror circuit (I assume that's what the 2X next to Mb2 means).
So if you know the voltage across the circuit, and the total circuit current as determined by the current mirror, then you can easily calculate its power dissipation.
P.S. This sounds like a problem that should be in the homework section.