Comment # 13 on bug 1023385 from Stefan Dirsch

Klaus, this may only work if CONFIG_DRM_AMDGPU_SI is set in kernel. Check with

  zcat /proc/config.gz | grep AMDGPU

Also I don't understand why adressing an issue with flickering should resolve a
crash in GNOME session startup.

I definitely vote for not enabling the support for AMD Southern Islands GPUs
in amdgpu driver.

config DRM_AMDGPU_CIK
        bool "Enable amdgpu support for CIK parts"
        depends on DRM_AMDGPU
        help
          Choose this option if you want to enable experimental support
          for CIK asics.

          CIK is already supported in radeon.  CIK support in amdgpu
          is for experimentation and testing.

As you can see this is considered experimental. And I doubt this will change
ever, since testings is going to decrease with SI GPUs getting older and older.
This is support for older, not newer GPUs! We really want to stay on radeon for
these, a well tested driver on these GPUs.