tests/gem_partial_pwrite_pread: don't trash gtt unnecessarily

On chips that don't have a unmappable gtt part it's utterly pointless.

Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/tests/gem_partial_pwrite_pread.c b/tests/gem_partial_pwrite_pread.c
index 57076cf..e00167f 100644
--- a/tests/gem_partial_pwrite_pread.c
+++ b/tests/gem_partial_pwrite_pread.c
@@ -124,7 +124,8 @@
 
 	drm_intel_gem_bo_unmap_gtt(tmp_bo);
 
-	if (bo->offset < num_trash_bos*1024*1024)
+	if (bo->offset < num_trash_bos*1024*1024 &&
+	    (IS_G33(devid) || intel_gen(devid) >= 4))
 		trash_aperture();
 
 	copy_bo(tmp_bo, bo);