UbuntuDrivers/detect.py: do not autoinstall the broadcom driver

The open driver should be a better solution now (LP: #1306928).
UbuntuDrivers/detect.py: autoinstall linux-firmware-nonfree

This will help now that we no longer autoinstall Broadcom's binary
driver.
gpu-manager.c: ignore CRT displays when using PRIME

This works around systems whose BIOS provides a ghost output (LP: #1296020).
Update changelog