When looking through the boot messages more closely than usual, I found the message
aug 10 12:01:53 mimmi kernel: [drm] drm_kms_firmware.edid_firmware is deprecated, please use drm.edid_firmware instead.
When trying to do as I was told, it was confusing since I couldn't find I used "drm_kms_firmware" when grep:ing through my configurations. It turns out I used the parameter drm_kms_helper.edid_firmware. If I understand how the code in drivers/gpu/drm/drm_kms_helper_common.c correctly, that is indeed the parameter being warned about. This might simply be a typo in the message, unless there is something I'm missing.
Thanks for the report, fix posted at http://firstname.lastname@example.org
Thanks for the report, fixed in drm-misc-next branch by
Author: Jani Nikula <email@example.com>
Date: Wed Aug 21 12:43:12 2019 +0300
drm: fix module name in edid_firmware log message