Joshuas comment #54 made me curious if it's a card specific issue. So I swapped
some cards around my boxes. I run both machines on Kernel 4.13.5.

on my PowerMac G5 11,2:
Radeon HD 5450: hits the bug
Radeon HD 6570: hits the bug
   Radeon X600: WORKS

on Athlon X4 845:
Radeon HD 5450: WORKS
Radeon HD 6570: WORKS
   Radeon X600: WORKS

The r380 Radeon X600 passes the ring test whereas the r600 cards fail:
# dmesg | grep -i drm
[    9.523830] [drm] radeon kernel modesetting enabled.
[    9.525292] [drm] initializing kernel modesetting (RV380 0x1002:0x5B62
0x1002:0x0F02 0x00).
[    9.595072] [drm] GPU not posted. posting now...
[    9.740409] [drm] Generation 2 PCI interface, using max accessible memory
[    9.740442] [drm] Detected VRAM RAM=128M, BAR=128M
[    9.740445] [drm] RAM width 64bits DDR
[    9.740785] [drm] radeon: 128M of VRAM memory ready
[    9.740789] [drm] radeon: 512M of GTT memory ready.
[    9.740810] [drm] GART: num cpu pages 131072, num gpu pages 131072
[    9.742591] [drm] radeon: 1 quad pipes, 1 Z pipes initialized
[    9.763583] [drm] PCIE GART of 512M enabled (table at 0x0000000088040000).
[    9.763705] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[    9.763708] [drm] Driver supports precise vblank timestamp query.
[    9.763818] [drm] radeon: irq initialized.
[    9.763825] [drm] Loading R300 Microcode
[   10.233486] [drm] radeon: ring at 0x0000000068001000
[   10.233545] [drm] ring test succeeded in 0 usecs
[   10.234665] [drm] ib test succeeded in 0 usecs
[   10.235279] [drm] Radeon Display Connectors
[   10.235284] [drm] Connector 0:
[   10.235287] [drm]   DVI-I-1
[   10.235289] [drm]   HPD1
[   10.235293] [drm]   DDC: 0x60 0x60 0x60 0x60 0x60 0x60 0x60 0x60
[   10.235296] [drm]   Encoders:
[   10.235299] [drm]     CRT1: INTERNAL_DAC1
[   10.235302] [drm]     DFP1: INTERNAL_TMDS1
[   10.308511] [drm] fb mappable at 0x880C0000
[   10.308521] [drm] vram apper at 0x88000000
[   10.308525] [drm] size 8294400
[   10.308527] [drm] fb depth is 24
[   10.308530] [drm]    pitch is 7680
[   10.372575] radeon 0001:06:00.0: fb0: radeondrmfb frame buffer device
[   10.372649] [drm] Initialized radeon 2.50.0 20080528 for 0001:06:00.0 on
minor 0

So I conclued this issue is produced by some interaction of the r600 DRM with
PowerPC-specific or Mac-PCIe-specific things?