backend/drm: Crash during Cog shutdown in gbm_dri_bo_destroy

[charlie-ht]

I'm consistently hitting at shutdown the following crash,

#0  0xb0483d60 in gbm_dri_bo_destroy (_bo=0x14e04f0) at ../src/gbm/backends/dri/gbm_dri.c:870
#1  0xafb9e308 in destroy_buffer (buffer=0x15383c0) at /home/cht/igalia/buildroot/output/build/cog-custom/platform/cog-platform-drm.c:121
#2  0xafb9f090 in clear_drm () at /home/cht/igalia/buildroot/output/build/cog-custom/platform/cog-platform-drm.c:149
#3  cog_platform_plugin_teardown (platform=<optimized out>) at /home/cht/igalia/buildroot/output/build/cog-custom/platform/cog-platform-drm.c:824
#4  0x00011dc0 in on_shutdown (launcher=<optimized out>, user_data=<optimized out>) at /home/cht/igalia/buildroot/output/build/cog-custom/cog.c:339
#5  0xb34bc014 in g_closure_invoke (closure=0x14659e8, return_value=0x0, n_param_values=1, param_values=0xbeedfaf8, invocation_hint=0xbeedfab4) at ../gobject/gclosure.c:810
#6  0xb34cd6e0 in signal_emit_unlocked_R (node=node@entry=0x1454b18, detail=detail@entry=0, instance=instance@entry=0x1457478, emission_return=emission_return@entry=0x0, 
    instance_and_params=instance_and_params@entry=0xbeedfaf8) at ../gobject/gsignal.c:3641
#7  0xb34d1d10 in g_signal_emit_valist (instance=instance@entry=0x1457478, signal_id=<optimized out>, detail=detail@entry=0, var_args=..., var_args@entry=...)
    at ../gobject/gsignal.c:3397
#8  0xb34d2128 in g_signal_emit (instance=instance@entry=0x1457478, signal_id=<optimized out>, detail=0) at ../gobject/gsignal.c:3453
#9  0xb35c962c in g_application_run (application=application@entry=0x1457478, argc=argc@entry=4, argv=argv@entry=0xbeedfda4) at ../gio/gapplication.c:2565
#10 0x00011b94 in main (argc=4, argv=0xbeedfda4) at /home/cht/igalia/buildroot/output/build/cog-custom/cog.c:491

On armv7l with master versions of cog, libwpe, wpebackend-fdo and WebKit 2.27.91. My hunch is that clear_gbo is stomping on something in the Mesa DRM stack that clear_drm gets unhappy about.