drm/i915: Allocate scratch page from stolen

With the last direct CPU access to the scratch page removed, we can now
allocate it from our small amount of reserved system pages (stolen
memory).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467390209-3576-10-git-send-email-chris@chris-wilson.co.uk