The warning message below has been displayed after upgrading Debian testing :
W: Possible missing firmware /lib/firmware/i915/kbl_guc_ver9_14.bin for module i915
W: Possible missing firmware /lib/firmware/i915/bxt_guc_ver8_7.bin for module i915
This issue has been referenced in the Debian bug tracking system : #857997
For now, this package does not provide the latest version of Kabylake GuC (9.14) and Broxton GuC (8.7) firmwares.
To remove this warning message, as a workaround, just download them and copy the blob files into the directory /lib/firmware/i915.
Finally, rebuild your initramfs :
update-initramfs -k `uname -r`-u