Just for the record: At this point, I can say that with
amggpu.vm_update_mode=3 4.17.2-ARCH runs at least for hours,
not only the minutes it runs without this option before crashing.

I cannot, however, say that above combination reaches the
some-days-between-amdgpu-crashes uptimes that 4.13.x reached -
in order to be able to test this, I would need S3 resumes to work,
which is subject to bug report 107065.

Without working S3 resumes, there is no way for me to test longer
uptimes because amdgpu consistently crashes (in any version I know
of) if I just let the system run but switch off the display, and I do
not want to keep the connected 4k TV switched on all day and night.