Calculating dBFS for I/Q
The sensible way here is to realize that each complex sample has a magnitude $|z|=\left\lvert \Re z + j \Im z\right\rvert = \sqrt{\left(\Re z\right)^2+\left(\Im z\right)^2}$. 

In your case, the maximum non-clipping amplitude is $2^{15}$ (the maximum-power samples are on a circle around the origin).

Therefore, you'd calculate the magnitude of a sample, and compare that to $2^{15}$ to get your relation to full scale.