drm/i915: Drop get/put_pages for scratch page

While discussing/reviewing __GFP_MOVEABLE behaviour and interactions
with our various page allocations on irc Chris brought up that the
scratch page isn't allocated as moveable, but we still grab/put a
reference to lock it in place. Which is unecessary.

So drop that.

Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>