Yeah, unfortunately that's a glitch in the most recent gpu bios update for rebar. If you want to enter bios connect via hdmi.
My brand new R10 5900 running 2.1.2 refuses to enter the bios vis F2 , F12 or windows advanced startup  I have tried everything and it just doesn’t enter into the bios gives a black screen and gets stuck until I power it off with the button 
Aurora R10, cannot access BIOS hitting F2? #2