You mean the capacitor is between the drain of the PMOS and ground... right?
Added:
What is about the gate? Sorry, you already said the PMOS is conducting
In case there is only C, then there is a transient dissipation till C is charged to the peak voltage. This transient power (energy) depends on the equivalent series resistance (impedance in general) between the supply and C.
Just a guess, if this circuit is a part of a controlled rectifier then there will be a load in parallel with C.