I have the same problem and its problem from AMD drivers 100% im waiting to they fix it
Its not driver problem,because no signal before operation system load.
I think its a combination pcb design(power draw)+bios from powercolor.
This thread is a huge glimmer of hope for me. I have a samsung tv and a Gigabyte G1 RX 480 8GB and I tend to clean reinstall windows twice a year. Every time the last several years I have been trying to solve this. It makes it impossible to turn off CSM. Everytime I do I get no video signal at boot, yet the computer is obviously just going about its day. Well not so obvious until recently. I found my HDD led was wired backwards (polarity) so now I can see there is a pulse. I also borrowed a monitor from a friend and what I used to think were no post situations were landing in the windows installer front page. I spent hours trying everything I could think of to boot into the UEFI USB install drive with CSM off. Hours. It actually was booting into the installer it's just not making a picture. If you turn on the PC with the HDMI pulled you can plug it in after the desktop loads and get an image. Not exactly practical. I'm going to try this 2 power cable business and will report back!