drm/nouveau: allow modeset module option to select 'headless mode'

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

diff --cc drivers/gpu/drm/nouveau/nouveau_drv.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/nouveau_drv.h
Simple merge

diff --cc drivers/gpu/drm/nouveau/nouveau_state.c
Simple merge