video: Add a way to write a partial string to the console

When writing multiple lines of text we need to be able to control which
text goes on each line. Add a new vidconsole_put_stringn() function to
help with this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/video.c b/test/dm/video.c
index c1b2a50..a9b2482 100644
--- a/test/dm/video.c
+++ b/test/dm/video.c
@@ -607,7 +607,8 @@
 	ut_assertok(video_get_nologo(uts, &dev));
 	ut_assertok(uclass_get_device(UCLASS_VIDEO_CONSOLE, 0, &con));
 	vidconsole_put_string(con, test_string);
-	ut_asserteq(12174, compress_frame_buffer(uts, dev, false));
+	vidconsole_put_stringn(con, test_string, 30);
+	ut_asserteq(13184, compress_frame_buffer(uts, dev, false));
 	ut_assertok(check_copy_frame_buffer(uts, dev));
 
 	return 0;