https://bugzilla.kernel.org/show_bug.cgi?id=48941
--- Comment #5 from Alex Deucher alexdeucher@gmail.com 2012-10-17 21:24:04 --- Created an attachment (id=83751) --> (https://bugzilla.kernel.org/attachment.cgi?id=83751) give each radeon_bl a unique name
This patch should fix the issue, however, you'll have to use the appropriate bl device depending on which gpu is active.