Attached is a v2 of the patch, for reference. I would appreciate if the original reporter or you tested it in lieu of your proposed patch and let me know if it fixes your
issue.

The patch works for me. echo 3 > /proc/sys/vm/drop_caches as well as rmmod radeon do not end up in a crash anymore. However, I have still no clue why one of these makes drm_open to fail. On rmmod radeon I get the following log messages. If don't know if the 'unpin not necessary' has anything to do with it.

[drm] radeon: finishing device.
radeon 0000:01:00.0: ffff88024e526c00 unpin not necessary
radeon 0000:01:00.0: ffff88024f2f6000 unpin not necessary
radeon 0000:01:00.0: ffff88024f2f6000 unpin not necessary
[TTM] Finalizing pool allocator
[TTM] Finalizing DMA pool allocator
[TTM] Zone  kernel: Used memory at exit: 0 kiB
[TTM] Zone   dma32: Used memory at exit: 0 kiB
[drm] radeon: ttm finalized
vga_switcheroo: disabled
[drm] Module unloaded

By the way, sometimes my r8169 ethernet controller does not survive suspend/hibernation (does not detect link). rmmod/modprobe helps. I don't know if this is related.