drm/i915: create dev_priv->dri1 dragon dungeon^W^W sub-struct

... and shove allow_batchbuffer in there. More dragons will
follow suit.

There's the curious case that we allow this for KMS ...

Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>