a lot out there on this issue  if you google it
 
hard to say all you can do is try things like new cable  or driver  maybe a win -10 thing is some crazy way ? [kinda old and may not be relevant ] 
http://www.tomshardware.com/forum/id-2601033/display-port-working-updating.html
 
thing is ports faliling or not working was a thing with 900 series    seems like gigabyte cards suffered the worse over all  . you should be able to google something like display port not working  980 ti / 970/ ect....  and see . 
 
 
best to google up  whats close to your issue and see what done for a fix   ,  but don't let your rma time run out  as a last ditch effort to resolve .  try another computer  [buddy ? ]   maybe with 7 on it to see if the same if so maybe the card for sure then .
 
also  I now recall like BENQ  had issues as well  with this  seems like I have see  2 or 3 like post where benq sent a new replacement monitor and all then was good  [seems there was 2 just here at evga on that  alone ]
 
https://forums.evga.com/980-Ti-Display-Port-Issue-m2350515.aspx
 
google display port not working gtx 980 ti and start reading up  try any fix  up till you got to try to rma the card  and hope for the best after that [plenty on it ]
 
if not may need to go with a dvi-I to display port / hdmi cable  to just bypass  using display port off the card ?
 
you see a lot of this when using multi monitor set ups as well ?
https://forums.geforce.com/default/topic/969477/gtx-980ti-displayports-no-longer-working/
 
https://forums.anandtech.com/threads/displayport-issues-with-980-970.2407184/
 
I did find interesting  that when no driver was used  / safe mode  the guy said it worked  but installing the NVidia driver it then failed ?   so did you update the driver and run in to this issue ?

RMA time.
900 series is notorious for DP ports going out.
:(

He already did all cable swapping, card swapping, machine swapping, different OS.
The DP ports dont work at all.
Its RMA time.

''I made sure to check the box for SLI''
 
just be sure you bring that up  there been guys that checked that box and was ignored  and evga sent them a non sli matching replacement card 
 
https://forums.evga.com/Looks-Like-An-RMA-upgrade-but-m2790283.aspx
 
 
I'd just make darn sure evga understands this  throughout the rma process

Both cards are being replaced with a 1070 Ti (08G-P4-5671-KR).  Is the HB bridge worth the $40?

I am currently using a flexible bridge.  I was using a EVGA Pro bridge on my previous motherboard, but it did not fit with the spacing of the new board.  I also have the rigid bridges that came with my board, just have to track them down.

My two replacement 1070 Ti's arrived yesterday.  I was pleased to see they were new sealed in the box, not repackaged.  They seem to run as well (if not better) than the 980 Ti's.  The Wither 3 seems to play well and PTGui stitches the large panoramic photos just as fast as the 980 Ti's did.  In all, I am happy with the upgrade...Just wish the warranty did not expire by July (original card warranty expiration).