drm/i915: Test creation of partial VMA

Mock testing to ensure we can create and lookup partial VMA.

v2: Named phases

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170213171558.20942-37-chris@chris-wilson.co.uk