drm/i915/selftest: annotate maybe unused but set variable unused

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

drivers/gpu/drm/i915/selftests/i915_gem_gtt.c

@@ -391,7 +391,7 @@ static void close_object_list(struct list_head *objects,
 			      struct i915_address_space *vm)
 {
 	struct drm_i915_gem_object *obj, *on;
-	int ignored;
+	int __maybe_unused ignored;
 
 	list_for_each_entry_safe(obj, on, objects, st_link) {
 		struct i915_vma *vma;