benchmarks: Add prime_lookup microbenchmark

Just a silly benchmark to stress prime_fd_to_handle and
prime_handle_to_fd.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/benchmarks/Makefile.sources b/benchmarks/Makefile.sources
index f69d4f1..3af54eb 100644
--- a/benchmarks/Makefile.sources
+++ b/benchmarks/Makefile.sources
@@ -15,6 +15,7 @@
 	gem_set_domain			\
 	gem_syslatency			\
 	kms_vblank			\
+	prime_lookup			\
 	vgem_mmap			\
 	$(NULL)