if im understanding you correctly, you have the original Bios now, so you will need to follow Bios Recovery process using 2nd GPU.
maybe there is something here
https://www.techpowerup.com/forums/threads/amd-ati-flashing-guide.212849/
guide from
@Solaris17
1.) let's reset your CMOS so we are on a clean slate by removing the battery on your board. This will allow us to make sure we are attempting flashes using the cardinal rule "DO NOT FLASH OVERCLOCKED"
2.) Do you have a bios switch on your GPU? If you do shut down your system now switch it to another position and attempt to boot your rig.
2b.) Let's restart the PC if all is well you are now booting off of the second bios on your GPU.
With your GPU working in the other bios switch position boot to your flash drive until you are at the root prompt and cursor. At this point it is assumed you have NOT ran any commands.
With your side panel off take this time to switch your bios switch to the opposite position (corrupted) with the system running. Make sure it is in the position it was on when it was corrupted.
To get the adapter number since using a secondary VGA card will change the previous command parameter you are familiar with.
With any luck you will have managed to fix your issue now. I will change and expand on this guide as time leisure and changes worth noting warrant it. If you have any questions additions or pictures of kittens please leave them below.
Now lets follow the flashing guide making sure at this point and time you FLASH BACK THE ORIGINAL, BACKUP, or CLOSEST COMPATIBLE BIOS!!
3.) If you do not have a BIOS switch remove the Affected GPU and boot using onboard video and in your BIOS set onboard Video as the default VGA adapter. In some cases with some motherboards it will be necessary to ENABLE the onboard GPU FIRST and THEN make it the default VGA adapter!
3b.) Shut down your system and re-insert the damaged GPU.
Start the system using onboard VGA and again using the flash guide above re-attempt to flash your GPU please take note to use
atiflash -i
To get the adapter number since using onboard VGA will change the previous command parameter you are familiar with.
4.) If you do not have onboard VGA please reproduce step
3.) using any spare VGA card compatible with your system. Boot using the compatible VGA card and set it as default.
4b.) Shut down your system and re-insert the damaged GPU.
Start the system using secondary VGA and again using the flash guide above re-attempt to flash your GPU please take note to use
atiflash -i
To get the adapter number since using a secondary VGA card will change the previous command parameter you are familiar with.
With any luck you will have managed to fix your issue now. I will change and expand on this guide as time leisure and changes worth noting warrant it. If you have any questions additions or pictures of kittens please leave them below.
(atleast im working in the Refinery 
)