benchmarks: Add a set-domain benchmark

Benchmark the overhead of changing from GTT to CPU domains and vice
versa. Effectively this measures the cost of a clflush, and how well the
driver can avoid them.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/benchmarks/.gitignore b/benchmarks/.gitignore
index 67afa18..4440de3 100644
--- a/benchmarks/.gitignore
+++ b/benchmarks/.gitignore
@@ -6,6 +6,7 @@
 gem_exec_trace
 gem_mmap
 gem_prw
+gem_set_domain
 gem_userptr_benchmark
 gem_wait
 intel_upload_blit_large