drm/i915: Add mock tests for GTT/VMA handling

Use the live tests against the mock ppgtt for quick testing on all
platforms of the VMA layer.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170213171558.20942-44-chris@chris-wilson.co.uk