Reported on the forums by user m_0_r_0_n (who really isn't one). This is an easy one-line patch that I will push into a new patchball shortly. I haven't looked closely yet, so I'm not sure what kernel versions are required to trigger this, but it was reported in 2.6.20, so it's probably an older compat function that was missed in one of the DRI commits. Looking at the current git, it seems this will need to go upstream, too. I'll post a comment when that's filed.
Upstream bug has been filed: https://bugs.freedesktop.org/show_bug.cgi?id=17595
Pushed 20080710 patchball 0.4 into portage.