drm/i915/selftests: fix NULL vs IS_ERR() check in mock_context_barrier()

The mock_context() function returns NULL on error, it doesn't return
error pointers.

Fixes: 85fddf0b ("drm/i915: Introduce a context barrier callback")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190321092451.GK2202@kadam

drivers/gpu/drm/i915/selftests/i915_gem_context.c

@@ -1620,8 +1620,8 @@ static int mock_context_barrier(void *arg)
 	mutex_lock(&i915->drm.struct_mutex);
 
 	ctx = mock_context(i915, "mock");
-	if (IS_ERR(ctx)) {
-		err = PTR_ERR(ctx);
+	if (!ctx) {
+		err = -ENOMEM;
 		goto unlock;
 	}