mm/memblock.c: fix bug in early_dyn_memhotplug

idx should be added after it is used, now it's added before
it is used. This is a bug, so fix it.

Change-Id: Ia875350b5cd30f9a8c1b57edac61163ef4e11cb1
Signed-off-by: Jiacheng Zheng <jiaczhen@codeaurora.org>