Comment 83 for bug 586325

Hrm. I don't seem to have asked whether this bug is still present when kernel modesetting is disabled, which is a bit of an oversight! Has disabling kernel modesetting and reverting to user modesetting been tried?

Adding “nomodeset” to the kernel command line will disable kernel modesetting. This will give a slightly worse boot experience - the splash screen will not be high-resolution, and will not seamlessly transition to X, but should otherwise function as normal.

If that sets the resolution correctly then we've got a workaround and a successful codepath to compare to.