[PATCH] drm/i915/selftests: silence a bogus spammy warning

[Chris Wilson]

Quoting Dan Carpenter (2017-06-14 10:14:52)
> These tests are reversed so it complains on every successful call and
> stays quiet for every failure.  Also we may as well preserve the error
> code.

The test is correct. The expectation here is that i915_gem_object_ggtt_pin()
fails and reports ENOSPC as we have already allocated the entire GGTT. If it
doesn't then we report the test failure.
-Chris