drm/i915/selftests: Free mock_i915->drm.mode_config

Even for the mock i915 device, we need to initialise the
drm.mode_config, as we may ultimately query whether there are any KMS
users deep in the bowels of some paths (e.g. eviction). As we initialise
drm.mode_config we must cleanup after ourselves!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171209210835.32609-1-chris@chris-wilson.co.uk