drm/i915: Embed the scratch page struct into each VM

As the scratch page is no longer shared between all VM, and each has
their own, forgo the small allocation and simply embed the scratch page
struct into the i915_address_space.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20160822074431.26872-2-chris@chris-wilson.co.uk


Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Acked-by: Mika Kuoppala <mika.kuoppala@intel.com>