x86: coreboot: Enable VIDEO_COPY

At least on modern machines the write-back mechanism for the frame buffer
is quite slow when scrolling, since it must read the entire frame buffer
and write it back.

Enable the VIDEO_COPY feature to resolve this problem.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>