Does LVDS always need 100 ohm differential impedance?
You might see if your flex vendor can provide 0.075 mm trace width. This capability is fairly common for rigid board vendors now, and my understanding is that flex has historically been able to support smaller features than rigid (but I don't design flexes regularly).

With 0.075 mm traces it's pretty easy to get 100 ohm differential:



That said, if your driver IC can handle an 80 or 85 ohm load, I don't see any reason not to use that instead. You'll get lower loss and probably more accurate characteristic impedance with the wider traces.