Dear Chris, your commit 9f267eb8d2ea ("drm/i915: Stop the machine whilst capturing the GPU crash dump") has popped up in today's linux-next and adds a select from DRM_I915 on STOP_MACHINE, which is not defined anywhere in Kconfig.
STOP_MACHINE has been removed a while ago (see https://lkml.org/lkml/2015/11/19/133). I assume that HOTPLUG_CPU could be used? I detected the issue with scripts/checkkconfigsymbols.py by diffing yesterday's and today's linux-next. Best regards, Valentin