arm: mm: correct printing when enabling VMALLOC_SAVING

keep the prev_reg so that we get correct printing if we have more
than two carve-out areas.

Change-Id: I931a091d3a9e19af36062e2a3fa15ec84ef5f7e4
Signed-off-by: Zhenhua Huang <zhenhuah@codeaurora.org>
Signed-off-by: Qingqing Zhou <qqzhou@codeaurora.org>