drm/i915/gtt: Reuse the read-only 64KiB scratch page and directories

If we can prevent stray writes from landing in the scratch page, we can
reuse the same page and same scratch PT for all contexts without fear of
information leaks and side-channels.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181029182721.29568-2-chris@chris-wilson.co.uk