I'm late to this thread, but I may have something useful to add. It may help someone else on a legacy system, so I figured I'd chime in.
Using a DP to VGA adapter is not an ideal solution for most people, but I guess if that is okay for the OP, who's to argue. Perhaps a DP to HDMI or DVI would also work. I've not tried that, but I did hear others that got it to work.
I believe theres an issue with the newer clover versions on legacy boards. I can successfully use clover v4934 with my rx570 card, but I get the same black screen during apple logo boot problem, that OP describes with his rx580, on clover v5120 (thats on DVI-D port only display connected). I tried debugging the issue with lilu and whatevergreen debug kexts and there is definitely an issue in kernel log with the AMD controller continually trying to reconnect to the gpu, so that's why the screen is black. Not sure what causes this. I do see in boot log a strange thing... i see "ATI: get_vram_size returned 0x0" only on clover 4934 where things work. but i see "ATI: get_vram_size returned 0x1" on the clover 5120 where it doesn't work. not sure why, but that may be significant.
btw: also, I had to do the kext patch for the amd9500 controller in clover to assign dynamic ports. Just using whatevergreen did not work, and still had black screen. and I used orinoco framebuffer. I messed with things like raddeinit, raddvi, adgpmod=pikera and other things to no avail. i'm using macpro5,1 as smbios as i think this best fits the motherboard, but maybe something else would work better.
anyway, hope that helps someone. these legacy boards are not used by many people anymore, so i think thats part of the problem where changes are made to something and not tested on legacy systems.