x86: fsp: graphics: Add some notes about the graphics info hob

On some platforms (eg: Braswell), the FSP will not produce the
graphics info HOB unless you plug some cables to the display
interface (eg: HDMI) on the board. Add such notes in the FSP
video driver.

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