If you can't or don't want to bisect, there are only 4 radeon driver commits
between 4.8.6 and 4.8.7, so it shouldn't take long to try manually reverting
each of those.
https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/commit/?h=linux-4.8.y&id=e136de5d733161fdfd203f23b448434170d189ea
seems like a good candidate, since it's clock related and explicitly references
your GPU in the code.