Step 1
Change grub to use NOMODESET as described in Installing Mint 14 or Mint 15 Compaq 6820s
Step 2
Add these lines to /etc/init.d/rc.local
modprobe -r radeon
modprobe -r drm
modprobe radeon modeset=1
To give something like this:
#! /bin/sh
### BEGIN INIT INFO
# Provides: rc.local
# Required-Start: $remote_fs $syslog $all
# Required-Stop:
# Default-Start: 2 3 4 5
# Default-Stop:
# Short-Description: Run /etc/rc.local if it exist
### END INIT INFO
modprobe -r radeon
modprobe -r drm
modprobe radeon modeset=1
Step 3
Reboot
Credits
I got the solution from this Redhat kernel thread, specifically Comment #104 so thanks to Lyonel Vincent.