I am late on the topic… but I have a solution that may be interesting for people with the same issue that will find this message.
TL,DR: flash the AMD Radeon V420 VBIOS on the MI50. Then it should run on a consumer motherboard 
I have got 3 AMD Radeon Instinct MI50 32 GiB. My purpose is to create a 3 GPU 96 GiB rig for a good price. So I ran for an older good quality motherboard with 3 PCIe x16 properly spaced: an ASRock Z170 Formula OC (a gaming motherboard).
When I received the hardware, the issue appeared: the mobo was not able to grant 32 GiB of MMIO addresses on the PCIe bus… and the MI50 is an industrial card, it requires a 32 GiB addressing block, and consumer motherboards do not expect such requirements. I have tried to improve the BIOS with fixes we can find on the Web (patches, decompiling and reinjecting the DSDT…).
No success. I found out that one MI50 card works on an ASUS ITX card I had (only one as there is a single PCIe slot). So I ordered a similar motherboard of the same period and brand with proper PCIe slots. I got an ASUS Z270-WS.
As expected, one GPU worked… but only one! The BIOS bootup hanged with Q-code error 96 with 2 or 3 cards (“issue on MMIO assignment”, meaning the PCIe bus cannot grant several times 32 GiB addressing). I tried BIOS improvements… when finally I got the computer booting with no video signal. I supposed the video was handled by the MI50… but the MI50 video output is disabled.
So I put back the original BIOS on the motherboard to get my integrated video back. In order to resume the investigations, I enabled the video output on MI50: I got another Video BIOS that is known to work on MI50 with video output enabled, AMD Radeon Pro V420.
After flashing one card, I discovered an interesting fact (with “lspci -vv -s …” under Linux): the MI50 was not claiming 32 GiB on the PCIe bus… but only 256 MiB (used as a window on the 32 GiB that all remain reachable).
So I flashed all 3 MI50 with the V420 VBIOS… and the rig is now working: Linux, amdgpu, and ROCm all fully detect the 3 GPU and the 96 GiB.
In short, there is an incompatibility between the industrial MI50 that requires 32 GiB addressing on the PCIe bus, and consumer motherboards that are not designed for that. When we move to the Video BIOS of a graphic card, things get right as a video board expects a consumer motherboard and starts less demanding on the bus addresses.
Note that if you have a more recent chipset than Z170/Z270, the V420 VBIOS will start at 256 GiB and do a ReBAR negociation with the motherboard for a larger window on VRAM (up to 32 GiB).
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