Of course it didn't make sense to turn it on, as it was already turned on
(DynPwr) in my case :) ). I can switch it off with vgaswitcheroo (or at least
to be reported as DynOff - I will check it later with powertop) when no
external output is connected. And what I missed in my previous comment - the
external monitor works out-of-box, which is nice progress.

Later on I will try to play with DRI_PRIME to enable also rendering with the
NVidia card.

What could be the side effects of:
> kernel: nouveau 0000:01:00.0: DRM: failed to create kernel channel, -22
?

And, in general, thanks for you suggestion with NvChipset :). If you would like
to test anything else on that hardware, please let me know.