What version of LR6 did you install? Are your graphics drivers up to date?
Lightroom 6.0 available from Amazon:
ASIN B00W5W7PJ0 Release date April 21, 2015
And we did try updating graphics driver, as well as running a beta AMD utility, with no change.
First, download and install the 6.7 update from here: Install Photoshop Lightroom
Then please check again - I just did on my system and it is there close to the end of the file (about 10 lines back from EOF).
After updating to 6.7 everything seems to work - the "useautobahn" flag is in the preferences file, and better yet lightroom hasn't crashed yet even with GPU enabled. Kind of surprised the download and install did not include the latest updates, but all better now. Thanks!
Happy it works for you now
The download installs the basic 6.0 and then can be directly updated to the latest patch level - patches are cumulative, which means that only the latest needs to be applied.
For further patches, there is an update function in the Help menu.