ANDROID: dma-buf: heaps: Allow the DMA-BUF system heap to be a module

This patch sets CONFIG_DMABUF_HEAPS_SYSTEM to tristate and exports the
symbol dma_heap_get_dev() to allow the DMA-BUF system heap to be a
module.

This change is intended to allow partners to continue using
device/SoC-specific optimizations in their system heap implementations
when they switch from ION to DMA-BUF heaps. The ION system heap was
built-in and partners were asked to override the system heap ID with
their own if they wanted to override the GKI system heap. This
per-vendor approach to override system heap will no longer be possible
with DMA-BUF heaps since Android S aims to restrict framework access to
DMA-BUF vendor heaps by only letting it access ABI-defined vendor heaps.

VTS tests will be created to ensure that the system heap is present
at /dev/dma_heap/system and behaves as expected of the system heap.

Bug: 175697666
Bug: 155218010
Bug: 172527615
Change-Id: Iabb24d9aedde308a9b15509781dd0e6b67353e99
Signed-off-by: Hridya Valsaraju <hridya@google.com>

drivers/dma-buf/dma-heap.c

@@ -277,6 +277,7 @@ struct device *dma_heap_get_dev(struct dma_heap *heap)
 {
 	return heap->heap_dev;
 }
+EXPORT_SYMBOL_GPL(dma_heap_get_dev);
 
 struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info)
 {

drivers/dma-buf/heaps/Kconfig

@@ -1,5 +1,5 @@
 config DMABUF_HEAPS_SYSTEM
-	bool "DMA-BUF System Heap"
+	tristate "DMA-BUF System Heap"
 	depends on DMABUF_HEAPS
 	help
 	  Choose this option to enable the system dmabuf heap. The system heap