Concerning my issue, which I now believe is unrelated, I tracked it down to
this commit in libdrm:

a02900133b32dd4a7d6da4966f455ab337e80dfc is the first bad commit
commit a02900133b32dd4a7d6da4966f455ab337e80dfc
Author: Emil Velikov <emil.velikov@collabora.com>
Date:   Tue May 15 17:29:44 2018 +0100

    xf86drm: introduce a get_real_pci_path() helper

    Introduce a helper which gets the real sysfs path for the given pci
    device.

    In other words, instead opening the /sys/dev/char/*/device symlink, we
    opt for the actual /sys/devices/pci*/*/

    It folds three (nearly identical) snprintf's and paves the way of adding
    extra devices (see next patch) a piece of pie.

    v2: use a caller (on stack) provided real_path (Eric)

    Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
    Tested-by: Robert Foss <robert.foss@collabora.com> (v1)
    Reviewed-by: Robert Foss <robert.foss@collabora.com> (v1)
    Reviewed-by: Eric Engestrom <eric@engestrom.ch> (v1)

:100644 100644 02da3e1f9b2ac5be41caf0178aefc07da018c945
51e00d23d90fd4a5217eb1aeae964140002cf417 M      xf86drm.c