Pull all of the video handling into a separate file, since a lot
more code will be added and such code would polute the board file.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Sean Cross <xobs@kosagi.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tim Harvey <tharvey@gateworks.com>