luv: Disable early boot graphics

Things don't work across the board right now, anyhow. People have even
experienced hangs, and there's no way we can diagnose those hangs
without displaying the test log on the screen.

It's not ideal, and going forward we definitely need to satisfy both
requirements (easy debugging and pretty graphics), but if need to pick
one or the other, easy debugging wins every time.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>