ion: add support for non-pool allocations

Currently it is difficult to test the real performance of
allocating from the ion system heap since the heap uses
a page pool.  Customers regularly encounter
scenarios where the pool is empty and memory has to
be allocated from the system.

Add support for the ION_FLAG_POOL_FORCE_ALLOC flag so that
the pool can be bypassed in order to measure the performance
of non-pool system heap allocations.

Change-Id: Ia89910abf52282a9696bd5abb9270930078c66a3
Signed-off-by: Liam Mark <lmark@codeaurora.org>