intel/perf: create a vtable entry for bo_unreference

In preparation for calling both Iris and i965 implementions from perf.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/intel/perf/gen_perf.h

@@ -191,6 +191,7 @@ struct gen_perf_config {
 
    struct {
       void *(*bo_alloc)(void *bufmgr, const char *name, uint64_t size);
+      void (*bo_unreference)(void *bo);
    } vtbl;
 };