Looking for older kernels I found 3.13-0.bpo.1-amd64 and voila, it fixed the
behavior.
Then I booted 3.14-2 and 3.16-1 and both worked again!
I compared the initramfs for 3.16-1 before and after and the only changes were
in etc/modprobe.d/radeon-kms.conf and etc/boottime.kmap.gz but copying those
changes by hand into the now-working initramfs did not duplicate the failure.