Comment 26 for bug 1161981

Some notes on the final patch (not attached here, but in ubuntu5 upload):
- tested on panda, confirmed to work
- tested on a normal (hybrid) laptop, previous versions had a bug which triggered ssp due to an incorrect offset, confirmed to work with the incorrect offset removed.
- tegra, was found to work by marvin24 in #ubuntu-x, using the set_drm_info fallback path.
- usb (udl) completely untested, I did an educated guess based on the kernel source. usb drivers do not have a busid, cf linux/drivers/gpu/drm/drm_usb.c drm_usb_set_busid, so busid is set to an empty string, rather than null. Looking at drmGetBusid in libdrm/xf86drm.c this would be the expected return value. xxv-modesetting works with path, rather than busid, so it would still work correctly.