benchmarks: Measure BLT performance

Execute N blits and time how long they complete to measure both GPU
limited bandwidth and submission overhead.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/benchmarks/.gitignore b/benchmarks/.gitignore
index 1c40a52..3937d46 100644
--- a/benchmarks/.gitignore
+++ b/benchmarks/.gitignore
@@ -1,3 +1,4 @@
+gem_blt
 gem_create
 gem_exec_ctx
 gem_exec_nop