amdgpu with a AMD Radeon 270X (Curacao XT): failed to link firmware kernel module

[pfwb]

I'm trying to use an AMD Radeon R9 270X (Curacao XT) with the amdgpu driver, which fails at boot with this log:

Mar 15 20:30:56 ORION kernel: [drm] amdgpu kernel modesetting enabled.
Mar 15 20:30:56 ORION kernel: drmn0: <drmn> on vgapci0
Mar 15 20:30:56 ORION kernel: vgapci0: child drmn0 requested pci_enable_io
Mar 15 20:30:56 ORION syslogd: last message repeated 1 times
Mar 15 20:30:56 ORION kernel: [drm] initializing kernel modesetting (PITCAIRN 0x1002:0x6810 0x174B:0xE271 0x00).
Mar 15 20:30:56 ORION kernel: [drm] register mmio base: 0xFDA80000
Mar 15 20:30:56 ORION kernel: [drm] register mmio size: 262144
Mar 15 20:30:56 ORION kernel: [drm] PCI I/O BAR is not found.
Mar 15 20:30:56 ORION kernel: [drm] BIOS signature incorrect 0 0
Mar 15 20:30:56 ORION kernel: ATOM BIOS: 113-1E27100-O4A
Mar 15 20:30:56 ORION kernel: [drm] vm size is 64 GB, 2 levels, block size is 10-bit, fragment size is 9-bit
Mar 15 20:30:56 ORION kernel: drmn0: failed to link firmware kernel module with mapped name: radeon_si58_mc_bin
Mar 15 20:30:56 ORION kernel: radeon/si58_mc.bin: could not load firmware image, error 2
Mar 15 20:30:56 ORION syslogd: last message repeated 1 times
Mar 15 20:30:56 ORION kernel: drmn0: failed to load firmware with name: radeon/si58_mc.bin
Mar 15 20:30:56 ORION kernel: drmn0: si_mc: Failed to load firmware "radeon/si58_mc.bin"
Mar 15 20:30:56 ORION kernel: drmn0: Failed to load mc firmware!
Mar 15 20:30:56 ORION kernel: [drm:amdgpu_device_ip_init] sw_init of IP block <gmc_v6_0> failed -2
Mar 15 20:30:56 ORION kernel: drmn0: amdgpu_device_ip_init failed
Mar 15 20:30:56 ORION kernel: drmn0: Fatal error during GPU init
Mar 15 20:30:56 ORION kernel: [drm] amdgpu: finishing device.
Mar 15 20:30:56 ORION kernel: device_attach: drmn0 attach returned 2

There's no radeon_si58_mc_bin.ko but there's amdgpu_si58_mc_bin.ko in /boot/modules. I'm using FreeBSD 12.1-RELEASE, with drm-kmod-g2019071 and drm-fbsd12.0-kmod-4.16.g20200221.