Currently VtsHalGraphicsComposer3_TargetTest is run with a crosvm
display with a connected display. Use the configurable vkms driver to
run VtsHalGraphicsComposer3_TargetTest with a disconnected display.
This job should help to catch errors that arise only in headless mode.
Signed-off-by: Deborah Brouwer <deborah.brouwer@collabora.com>