Seems to be fixed by either 4.4.2 or 4.4.3.  I haven't experienced the issue in
a month now since upgrading from 4.4.1.  I can only assume this was indeed a
duplicate of bug 91268 as Michel guessed, and I was somehow wrong in thinking
that Slackware's 4.4 and 4.4.1 kernels contained the commit from that bug
(96ea47c0ec8c012509116bee8c57414281428fc4).

Thanks a bunch to the radeon devs for all your work.

David

*** This bug has been marked as a duplicate of bug 91268 ***

What	Removed	Added
Status	REOPENED	RESOLVED
Resolution	---	DUPLICATE