I went the easy way now. I built nouveau as a module that is loaded later and I
see

[    5.323010] nouveau  [  PGRAPH][0000:01:00.0] using external firmware

in my dmesg output :-) I will stay with the "nouveau as a module" solution
until I am sure, that the external firmware solves the problem. Then I can
still figure out how to make UEFI and initrd work together.

I will come back and report my results, but this may take its time, because the
crash only occurs once in a while.