amdgpu driver fails to initialize on ppc64le in 7.0-rc1 and newer

[Karl Schimanek]

Hi Ritesh,
Dan,

I can reproduce the issue on my system running Chimera Linux with 4K 
page size.

System details:
- Machine: RaptorCS Blackbird (POWER9, PowerNV)
- GPU: Sapphire Pulse Radeon RX 5500 XT (Navi 14)
- Kernel: 7.0-rc1

The failure is identical, ending in:

 > amdgpu ... (-12) failed to allocate kernel bo
 > amdgpu ... amdgpu_device_wb_init failed -12
 > amdgpu ... Fatal error during GPU init

I also see:

 > iommu: 64-bit OK but direct DMA is limited by 0
 > dma_iommu_get_required_mask: returning bypass mask 0xfffffffffffffff

This is with 4K pages, so it does not seem to be limited to 64K page 
configurations.

Regards,
Karl