FROMLIST: dma-buf: system_heap: Add a system-uncached heap re-using the system heap

This adds a heap that allocates non-contiguous buffers that are
marked as writecombined, so they are not cached by the CPU.

This is useful, as most graphics buffers are usually not touched
by the CPU or only written into once by the CPU. So when mapping
the buffer over and over between devices, we can skip the CPU
syncing, which saves a lot of cache management overhead, greatly
improving performance.

For folk using ION, there was a ION_FLAG_CACHED flag, which
signaled if the returned buffer should be CPU cacheable or not.
With DMA-BUF heaps, we do not yet have such a flag, and by default
the current heaps (system and cma) produce CPU cachable buffers.
So for folks transitioning from ION to DMA-BUF Heaps, this fills
in some of that missing functionality.

There has been a suggestion to make this functionality a flag
(DMAHEAP_FLAG_UNCACHED?) on the system heap, similar to how
ION used the ION_FLAG_CACHED. But I want to make sure an
_UNCACHED flag would truely be a generic attribute across all
heaps. So far that has been unclear, so having it as a separate
heap seemes better for now. (But I'm open to discussion on this
point!)

This is a rework of earlier efforts to add a uncached system heap,
done utilizing the exisitng system heap, adding just a bit of
logic to handle the uncached case.

Feedback would be very welcome!

Many thanks to Liam Mark for his help to get this working.

Pending opensource users of this code include:
* AOSP HiKey960 gralloc:
  - https://android-review.googlesource.com/c/device/linaro/hikey/+/1399519
  - Visibly improves performance over the system heap
* AOSP Codec2 (possibly, needs more review):
  - https://android-review.googlesource.com/c/platform/frameworks/av/+/1360640/17/media/codec2/vndk/C2DmaBufAllocator.cpp#325



Signed-off-by: John Stultz <john.stultz@linaro.org>
Link: https://lore.kernel.org/lkml/20201110034934.70898-8-john.stultz@linaro.org/
Bug: 170887642
Change-Id: I56cb3acf58546c7dfd423a2ce432e3d6d7fd7a69