Stopping Dell P2715Q monitor flickers with MacBook Retina 12" (2015)
This may be good or bad news deepening on how you look at it, but it’s the Dell 4K display, not your Mac. Both Dell model numbers 2715Q and 2815Q are notorious for this kind of flickering when used with MacBooks. If you search around on Amazon and other sites you will find similar complaints.

We purchased 4 of the Dell 4K P2715Q monitors where I work, and we returned ALL of them in the end because they exhibited this exact flickering behavior (as well as the inability to properly run at 60Hz).

It didn’t matter if we connected different types of Macs, or used different connectors. If we tried to drive these monitors at full resolution we saw this problem.

Dell told us the same thing when we called support (and we have the good support where I work), and we still never succeeded in making them work.

If you do find a solution with your Mac that makes this monitor work properly I will be happy to learn about it.

EDIT: For the benefit of anyone coming to this answer and wondering about other’s who’ve seen the same problem. Check out Apple’s forum posts reporting the same problems with the P2715Q.