drm/i915: Exercise manipulate of single pages in the GGTT

Move a single page of an object around within the GGTT and check
coherency of writes and reads.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170213171558.20942-45-chris@chris-wilson.co.uk