dm: video: Flush the cache after a puts()

This helps keep the display consistent. puts() is used when printing the
prompt, so is a useful way to make sure the current display contents is
visible.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Tom Warren <twarren@nvidia.com>