bios: vesa: Guard setting vesa mode with CONFIG_FRAMEBUFFER_SET_VESA_MODE

If CONFIG_FRAMEBUFFER_SET_VESA_MODE is not set, don't switch
graphics card to VESA mode. This applies to both native mode
and emulator mode of running the VGA BIOS.

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