video: bochs: Set the frame buffer size per configuration

At present the uclass stored frame buffer size is set to a hard
coded value, but we can calculate the correct value based on what
is configured.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>  # qemu-x86_64