I do not understand why you have:
In your variational form. You should pin the pressure at a single degree of freedom, see: Pointsource on specific points python - #2 by kamensky
There are various sources that have already considered solving the Navier stokes equation with dolfin, including:
Chapter 3.4
You can also find a similar problem in:
https://www.sciencedirect.com/science/article/abs/pii/S0045782520303145
Solving the Taylor-Green vortex problem.
The source code is available at: