drm/i915: Test partial mappings

Create partial mappings to cover a large object, investigating tiling
(fenced regions) and VMA reuse.

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/20170213171558.20942-19-chris@chris-wilson.co.uk