Some drivers need an explicit sync method to flush the virtual
framebuffer to the display. It is called fb_flush().

fb_flush() gets called on fbc_putc, on fb_close and in the pattern cycle
in the fbtest command.

Signed-off-by: Bastian Stender <bst@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>