e to avoid a forward
declaration. The new scratch page is not yet used, though.

v2: Refactor out scratch page teardown.  Suggested by Chris Wilson on
irc. This makes it clear what's going on and results in a nice
symmetry between setup and teardown.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>