drm/i915: Really share scratch page

A previous patch had set up the ppgtt and ggtt to use the same scratch
page, but still kept around both pointers. Kill it, it's not needed and
gets in our way for upcoming cleanups.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>