Just to say that I fixed this issue today.I have the same issue here.
All drivers installed OK, latest (AMD) graphics drivers installed, the Oculus app tells me that everything is connected & happy - but can't get anything to actually display in the DK2 headset.
I also have a message saying that my computer is below spec & the DK2 is unsupported. I'm wondering if it's complaining about my AMD 7870 graphics card.
My DK2 hasn't been used for a (fairly long) while, and what seems to have happened is that the HDMI cable has somehow busted itself. I replaced it, and everything sprang into life.
So if you have a similar issue (everything works except video on the headset) try replacing the cable. To open it, push at the bottom of the panel at the front, and slide up.