Comment # 3 on bug 1168383 from Giacomo Comes

Already fixed upstream. Here is the conversation I had on intel-gfx:

>> Should hopefully be fixed with
>> commit 2bdd4c28baff ("drm/i915/display: Fix mode private_flags
>> comparison at atomic_check")
>> 
>> Stable folks, please pick that up for 5.6.x stable releases.
>
>I can confirm that the commit indeed solves the problem I have.
>It should go in the stable 5.6.x release ASAP.

Hopefully the fix should land in the stable tree very soon.
When it happens, I'll close this bug.