dm: video: Sync display on backspace

We should sync the display (e.g. flush cache) when backspace is pressed
to ensure that the character is erased correctly.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Anatolij Gustschin <agust@denx.de>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Tested-on: Beaver, Jetson-TK1