ion: msm: Add support for ION heaps to implement prefetch and drain

Allow the ION heaps to implement the prefetch and drain
operations. This allows clients to prefetch memory to be able
to reserve memory for fast ION allocations at a later point
in time, as well as unreserve the memory.

Change-Id: I144a4bc45b8b4f28974734d2aa37692db8e169a1
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>