The sudo dmseg command shows the warning Possible missing firmware /lib/firmware/ast_dp501_fw.bin for module ast. This relates to the ASpeed BMS VGA graphics microcode. The simple solution seems to be be to install the non-free microcode blob using sudo apt install firmware-ast.

Package: firmware-ast (20140808-8) [non-free-firmware Ubuntu 15.10 Server; W: Possible missing firmware /lib/firmware/ast_dp501_fw.bin for module ast

Linux Upgrade (Debian) - Possible missing firmware problem [duplicate

Repository index of firmware blobs for use with the Linux kernel