common: Only mark malloc initialized after mem_malloc_init

Instead of marking malloc as initialized as soon as relocation is done,
defer it until after we call mem_malloc_init. This ensures that malloc
initialization is done before we switch away from simple_malloc, and
matches the SPL behavior.

Fixes: c9356be3 ("dm: Split the simple malloc() implementation into its own file")
Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>