It seems to involve mesa package. I have recompiled radeonsi_dri.so, maybe now someone can make sense out of it.
(EE) Backtrace: (EE) 0: /usr/lib/xorg/Xorg (OsLookupColor+0x139) [0x5609d99fc889] (EE) 1: /lib/x86_64-linux-gnu/libpthread.so.0 (funlockfile+0x50) [0x7fe8a6ea0f8f] (EE) 2: /lib/x86_64-linux-gnu/libc.so.6 (memcpy+0x2d7) [0x7fe8a6d5a387] (EE) 3: /usr/lib/x86_64-linux-gnu/dri/radeonsi_dri.so (si_buffer_subdata+0xa4) [0x7fe8a5524098] (EE) 4: /usr/lib/x86_64-linux-gnu/dri/radeonsi_dri.so (pipe_buffer_write+0x46) [0x7fe8a54d3233] (EE) 5: /usr/lib/x86_64-linux-gnu/dri/radeonsi_dri.so (si_pm4_upload_indirect_buffer+0x193) [0x7fe8a54d3ac7] (EE) 6: /usr/lib/x86_64-linux-gnu/dri/radeonsi_dri.so (si_init_config+0x775) [0x7fe8a54ebeab] (EE) 7: /usr/lib/x86_64-linux-gnu/dri/radeonsi_dri.so (si_init_state_functions+0x301) [0x7fe8a54eb479] (EE) 8: /usr/lib/x86_64-linux-gnu/dri/radeonsi_dri.so (si_create_context+0x648) [0x7fe8a54d0ed9] (EE) 9: /usr/lib/x86_64-linux-gnu/dri/radeonsi_dri.so (radeonsi_screen_create_impl+0xf57) [0x7fe8a54d2d93] (EE) 10: /usr/lib/x86_64-linux-gnu/dri/radeonsi_dri.so (amdgpu_winsys_create+0x472) [0x7fe8a559de54] (EE) 11: /usr/lib/x86_64-linux-gnu/dri/radeonsi_dri.so (radeonsi_screen_create+0x71) [0x7fe8a54d2faf] (EE) 12: /usr/lib/x86_64-linux-gnu/dri/radeonsi_dri.so (pipe_radeonsi_create_screen+0x24) [0x7fe8a53e6a57] (EE) 13: /usr/lib/x86_64-linux-gnu/dri/radeonsi_dri.so (pipe_loader_drm_create_screen+0x3a) [0x7fe8a54c787c] (EE) 14: /usr/lib/x86_64-linux-gnu/dri/radeonsi_dri.so (pipe_loader_create_screen+0x52) [0x7fe8a54c6bd5] (EE) 15: /usr/lib/x86_64-linux-gnu/dri/radeonsi_dri.so (dri2_init_screen+0xcd) [0x7fe8a53ee2e2] (EE) 16: /usr/lib/x86_64-linux-gnu/dri/radeonsi_dri.so (driCreateNewScreen2+0x1ed) [0x7fe8a53e7d7e] (EE) 17: /lib/x86_64-linux-gnu/libgbm.so.1 (gbm_format_get_name+0x189e) [0x7fe8a6455d0e] (EE) 18: /lib/x86_64-linux-gnu/libgbm.so.1 (gbm_format_get_name+0x1c23) [0x7fe8a64563b3] (EE) 19: /lib/x86_64-linux-gnu/libgbm.so.1 (gbm_create_device+0x57) [0x7fe8a64525e7] (EE) unw_get_proc_name failed: no unwind info found [-10] (EE) 20: /usr/lib/xorg/modules/drivers/amdgpu_drv.so (?+0x0) [0x7fe8a646d650] (EE) 21: /usr/lib/xorg/Xorg (InitOutput+0xbac) [0x5609d98deb5c] (EE) 22: /usr/lib/xorg/Xorg (InitFonts+0x1cf) [0x5609d98a17df] (EE) 23: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xeb) [0x7fe8a6cc6b6b] (EE) 24: /usr/lib/xorg/Xorg (_start+0x2a) [0x5609d988b67a] (EE) (EE) Illegal instruction at address 0x7fe8a6d5a2c7