edid: fill reserved bits fields in cb_framebuffer

If it's a 4 byte format (as per documentation), there
are some reserved bits, so let's mark them as such...

Change-Id: I50f12cfff2c9bb9d082a5f3c3ac54c0d514d862b
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/7674
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>