No chance for us. 8-)
Irregardless of the flags in ID definition, amdgpu driver tries to map 5th memory region for mmio - Linux/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c - Linux Cross Reference - Free Electrons
and 8970M has only the following regions:
Region 0: Memory at e0000000 (64-bit, prefetchable) [size=256M]
Region 2: Memory at f7b00000 (64-bit, non-prefetchable) [size=256K]
Region 4: I/O ports at e000 [size=256]
(verify with lspci -vvv)
Technically, as the card is GCN 1st gen, CHIP_TOPAZ may have suited (others are gen2-4 and cpus), but without controlling mmio, it all is kind of pointless.
We need to get "official" design paper for the card to make it work...