I have taken the time to find the source of the message. It is, indeed, from inteldrm(4), in /usr/src/sys/dev/pci/drm/i915/intel_sprite.c. The error message was added as part of revision 1.8 to this module. The log message states:
Code:
revision 1.8
date: 2017/07/01 16:14:10; author: kettenis; state: Exp; lines: +594 -588; commitid: KnwRPOZok9A30HI4;
Update inteldrm(4) to code based on Linux 4.4.70. This brings us support for
Skylake and Cherryview and better support for Broadwell and Valleyview. Also
adds MST support. Some tweaks to the TTM code and radeondrm(4) to keep it
working with the updated generic DRM code needed for inteldrm(4).