I've reproduced this issue on amd-staging-drm-next and have attached a UMR wave
and gfx ring dump, along with a new dmesg.  To clarify, this issue also
prevents me from rebooting/shutting down my computer, and I am forced to hold
the power button.

Here are the version strings of the relevant software I'm running:

Kernel: amd-staging-drm-next (commit: d2d07f246b126b23d02af0603b83866a3c3e2483)
Mesa: 18.3.1
Xorg: 1.19.6
UMR: 016bc2e93af2cac7a9bd790f7fcacb1ffdadc819

This is my first attempt at using UMR to get information about this system
hang.  I'm essentially just copying what Andrey Grodzovsky suggested in a
previous thread[0].

Here are the umr commands used to gather the information:

Waves dump: umr -i 1 -O verbose,halt_waves -wa

GFX ring dump: umr -i 1 -O verbose,follow -R gfx[.]

GFX info: umr -i 1 -e

I've attached the output of these to the bugzilla report.

[0] https://lists.freedesktop.org/archives/amd-gfx/2018-December/029790.html