First off, while I doubt that this has been fixed, it would be best to try the
latest drm-next, which has some additional nouveau fixes. Assuming the issue
still isn't resolved, could you boot with

nouveau.debug=PDISP=debug,I2C=debug,VBIOS=trace,DRM=debug drm.debug=0xe

And go through a DPMS cycle (and then powercycle the monitor to get it to turn
it back on), taking care to note where in the log the various (external) events
happen. I think you can actually just write to /dev/kmsg directly.