drm/ttm: fix small memory leak in ttm_memory.c

I moved the allocation until after the check for (si->totalhigh == 0).

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-By: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>