That commit (and v3.13.3 in general) are contained in the Ubuntu 3.13.0-10.30 upload, which would imply that fix was in the kernel in the -release pocket, in Ubuntu 3.13.0-24.46, and it cirtainly seems to be correctly applied there.
That said it does appear some other fixes have been applied in this area which may well have reenabled it for some of these cards. It appears some further investigation is needed, to determine why the fixes for this have not worked and to feed this back to stable.
@Michael -- the fix you propose for the issue was applied to 14.04 in the upstream stable update v3.13.3, the commit below:
commit 318729d307dc565 a1c44bb02a8710b 5a4f0b4037
Author: Alex Deucher <email address hidden>
Date: Sat Jan 11 10:55:55 2014 -0500
drm/radeon: disable dpm on BTC
commit 919cf555c04e16d afb1fba56904eb2 3889a812c3 upstream.
That commit (and v3.13.3 in general) are contained in the Ubuntu 3.13.0-10.30 upload, which would imply that fix was in the kernel in the -release pocket, in Ubuntu 3.13.0-24.46, and it cirtainly seems to be correctly applied there.
That said it does appear some other fixes have been applied in this area which may well have reenabled it for some of these cards. It appears some further investigation is needed, to determine why the fixes for this have not worked and to feed this back to stable.