tests/gem_cs_prefetch: clarify what it tests

I've forgotten ;-)

tests/Makefile.am

@@ -18,6 +18,7 @@ TESTS_progs = \
 	gem_basic \
 	gem_cacheing \
 	gem_cpu_concurrent_blit \
+	gem_cs_tlb \
 	gem_gtt_concurrent_blit \
 	gem_exec_nop \
 	gem_exec_blt \

tests/gem_cs_prefetch.c

@@ -31,6 +31,9 @@
  * Historically the batch prefetcher doesn't check whether it's crossing page
  * boundaries and likes to throw up when it gets a pagefault in return for his
  * over-eager behaviour. Check for this.
+ * 
+ * This test for a bug where we've failed to plug a scratch pte entry into the
+ * very last gtt pte.
  */
 #include <stdlib.h>
 #include <stdio.h>