drm/nouveau: headless mode by default if pci class != vga display

This is to prevent nouveau from taking over the console on headless boards
such as Tesla.

Backport of upstream commit: e412e95a268fa8544858ebfe066826b290430d51

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>