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.

Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>
[jstultz: modified patch to apply to staging directory]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Conflicts:
	drivers/staging/android/ion/ion_chunk_heap.c
	drivers/staging/android/ion/ion_heap.c
	drivers/staging/android/ion/ion_priv.h
	drivers/staging/android/ion/ion_system_heap.c