amdgpu.ko crashes on Radeon Vega

[George Mitchell]

I have (per dmesg): AMD Ryzen 3 2200G with Radeon Vega Graphics.
(pciconf -lv reports:
vgapci0 at pci0:6:0:0:     class=0x030000 card=0xd0001458 chip=0x15dd1002 
rev=0xc8 hdr=0x00
     vendor     = 'Advanced Micro Devices, Inc. [AMD/ATI]'
     device     = 'Raven Ridge [Radeon Vega Series / Radeon Vega Mobile
Series]')
I am running FreeBSD 12.2-RELEASE-p6 r369558.
I built amdgpu.ko from drm-fbsd12.0-kmod-4.16.g20201016_1.
When I boot single-user, with only kernel, vboxdrv.ko, and sem.ko
loaded, and I type "kldload amdgpu", I get about six or seven lines
of output, ending with "pci i/o bar is not found."  After a couple
more seconds, the screen goes blank and the machine is as frozen as
can be, and I have to cycle power to recover.  I've tried all four
combinations of hw.vga.textmode and hw.syscons.disable, but it
crashes each time.  Help!                              -- George

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