GPU to watch 4k 60fps content (no games)

AMD or Nvidia are both fine, just something I could use in an htpc (trying to futureproof a bit here).

Anything with DisplayPort 1.2 or HDMI2.0 Out

On a 4K tv you have to use nVidia... unless you have a Panasonic 4K with dp.

Even the new AMD GPUs don't have HDMI 2.0 - No AMD card has 2.0 and no converter cable exists even on Fury X

The cheapest is the 960 with HDMI 2.0