Prepare for re-enabling -Wunused-but-set-variable.

The variable is indeed 'unused' as the name suggests, but we can't just
drop it because i915_vma_unbind_unlocked() is annotated
__must_check. Apparently the selftest does not really need to check the
value.
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e7654682f6bd6a9f6af74f4b6eb5fff7b527e412.1685119007.git.jani.nikula@intel.com