Setting resolution to 4K kills nouveau ... but how do I recover?

[Richard W.M. Jones]

I've got a new machine with an nVidia card and an external 4K display.
For whatever reason selecting 4K in the graphical display resolution
tool causes the Nouveau driver to crash.  After this I can no longer
log in because even after a reboot it tries to select 4K again.

How can I (from ssh) change the display resolution back to something
lower?

Rich.