Hey, I'm not sure if you found a solution yet, but I had the same problem you are and I think i might have found a temp solution. This is assuming you can boot to your sierra successfully through only integrated intel graphics and not through the 7850. You can see my post
here for my problem.
So assuming you have clover UEFI installed, download and extract the two files below. One is a modified amdX4000 kext i found from
here and the other is a program called kext wizard.
1. open kext wizard and select installation and browse to your location of the downloaded amdX4000 and make sure to put a checkmark in the "backup kext that will be replaced" before clicking install. A backup copy will be placed on desktop. Everything else can be left alone.
2. Select the maintenance tab and check mark S/L/E and click execute. You always want to repair before you reboot your system with any kext edits.
Now reboot your system with one cable to your 7850 and another to integrated intel graphics and it should boot up past the black screen/apple logo glitch freeze. The main login screen background will be blurry but it will boot in. Mine rebooted on first reboot and then went through on second.
I tried searching around a lot and finally found this method to work, so thought i'd share it. Good luck.
