Sorry for necroposting but I'm currently going through the exact same problem.  
Same mobo, works fine with onboard VGA but won't output anything through GPU (sapphire rx470 nitro 8Gb).  

I tried fullzero's solution, doing a full power cycle and retrying with only one GPU in the x16 slot without a riser but still no luck for me.  

Is there anything else I can try before sending this back to the vendor?
Delaying some more would suck big time... 

Edit : Ok, I managed to boot with the GPU plugged in by going back and forth in the BIOS and setting the video out device to "onboard".
When I go in window's device manager, the GPU is recognized as "Microsoft Basic Display Adapter" with the status:
 "This device is not working properly because Windows cannot load the drivers required for this device "code 31) The driver trying to start is not the same as the driver for the POSTed display".
I then installed AMD's drivers and rebooted. Now I do have an output on that card, locked down to 640x480, for some reason... 
Not sure where to go from here.