gpu: ion: Refactor the code to zero buffers

Refactor the code in the system heap used to map and zero the buffers
into a seperate utility so it can be called from other heaps.  Use it from
the chunk heap.

Change-Id: I706341ae42b80bc4aae8a8614b4f73435bbf05d9
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>