drm/i915: Mock a GGTT for self-testing

A very simple mockery, just a random manager and timeline. Useful for
inserting objects and ordering retirement; and not much else.

v2: mock_fini_ggtt() to complement mock_init_ggtt().

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/20170213171558.20942-7-chris@chris-wilson.co.uk