[new uImage] Don't pass kdb to ramdisk_high since we may not have one

We don't actually need the kdb param as we are just using it to get
bd->bi_memsize which we can get from gd->bd->bi_memsize.  Also, if we
boot via OF we might not actually fill out a kdb.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Marian Balakowicz <m8@semihalf.com>