pmem: Allocate physical memory from reserved fmem

If a heap is marked as reusuable (i.e. memory is to be gotten from
fmem) the base used from allocation should not come from
allocate_contiguous_memory but from fmem_get_info instead. Otherwise,
the memory allocated isn't actually coming from fmem.

Change-Id: If9737c93df3bd34e9cac9585a4dbbc8993562c5f
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>