drm/i915/gtt: Make scratch page i915_page_dma compatible

Lay out scratch page structure in similar manner than other
paging structures. This allows us to use the same tools for
setup and teardown.

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>