drm/i915: Fix an error checking test

'dma_buf_vmap' returns NULL on error, not an error pointer.

Fixes: 6cca22ede8 ("drm/i915: Add some mock tests for dmabuf interop")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: http://patchwork.freedesktop.org/patch/msgid/20170627053854.21152-1-christophe.jaillet@wanadoo.fr
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/selftests/i915_gem_dmabuf.c

@@ -246,9 +246,9 @@ static int igt_dmabuf_export_vmap(void *arg)
 	i915_gem_object_put(obj);
 
 	ptr = dma_buf_vmap(dmabuf);
-	if (IS_ERR(ptr)) {
-		err = PTR_ERR(ptr);
-		pr_err("dma_buf_vmap failed with err=%d\n", err);
+	if (!ptr) {
+		pr_err("dma_buf_vmap failed\n");
+		err = -ENOMEM;
 		goto out;
 	}