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: If17f7ee74bbb1e2853bf09cf1764793ec45d8301
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>
Git-commit: 7994db16fbf60c3ab6292e5e42831a0eefa500b6
Git-repo: https://android.googlesource.com/kernel/common


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