I did some more testing with kernel 4.8rc5 and it seems it is not related to X after all. Cold boot to console has exactly the same issues as cold boot to X (screen blanks). The last message I see on the screen is “fb: switching to amdgpudrmfb from EFI VGA” then it turns off and on again but it is blank. So far console didn't blank after successful start like X does unless I suspended it.