https://bugs.freedesktop.org/show_bug.cgi?id=76286
--- Comment #5 from Gregory Land landjgregory@gmail.com --- I am having a similar problem with my 4870. If i disconnect my second monitor I can boot with radeon.dmp = 1. If I reconnect the monitor again it will lock up shortly after boot.
A bisect brings up this commit https://github.com/torvalds/linux/commit/ab70b1dde73ff4525c3cd51090c233482c5... Which makes sense since this commit enabled dpm by default on radeon x7xx series cards.