x86: minnowmax: Drop screen resolution to 1024x768

This seems like a more reasonable resolution for this board, since it is
quite slow. It also allows it to work with a 5" LCD display in my lab.

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