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: I494a1fdd84c2be40ce86d9c38adcbb809081672f
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>
Git-commit: a10359764fc8ce7a76c7c83552c1a09a5e673d51
Git-repo: https://android.googlesource.com/kernel/common


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