x86: fsp: video: Allocate a frame buffer when needed

When the copy framebuffer is in use, we must also have the standard U-Boot
framebuffer available. Update the FSP driver to support this.

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