gpu: ion: Refactor common mapping functions out of system heap

The system heap contained several general purpose functions to map
buffers to the kernel and userspace.  This patch refactors those
into ion_heap.c so they can be used by other heaps.

Change-Id: If64591798bdc2c248bf9064ace2c927909d7adb8
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>