I noticed that in the broken kernel, plymouth does not switch on the external monitor. Normally plymouth shows a cloned display.
Looking at the remaining commits, there was a merge of tag "drm-intel-next-2017-07-17" which had a very large number of i915 commits. You may want to manually choose commits either side of that merge, to hurry things along, since I don't think git-bisect will descend into the branch until the merge commit is fully isolated.
b9c55b6e2cc4369 b0688961fa5de0e 057f3ec0c4: fixed
I noticed that in the broken kernel, plymouth does not switch on the external monitor. Normally plymouth shows a cloned display.
Looking at the remaining commits, there was a merge of tag "drm-intel- next-2017- 07-17" which had a very large number of i915 commits. You may want to manually choose commits either side of that merge, to hurry things along, since I don't think git-bisect will descend into the branch until the merge commit is fully isolated.