drm/nouveau: reserve VGA area for the moment

This is to prevent things such as GART tables and other important GPU
structures being allocated there before we take over fbcon ourselves.

This is more of a workaround for the moment, a better solution will
require some more invasive changes, but it'll be done at some point.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>