ion: msm: plumb ION_HEAP_TYPE_DMA to core Ion

ION_HEAP_TYPE_DMA has made its way into core Ion since it was originally
added to the MSM Ion code base. Move some of the hooks and function
declarations out of MSM-specific code and into core Ion. There are still
some differences in the heap implementations themselves that will need
to be rectified later.

Change-Id: I44a4aa3a48645072cee0f2d0718ce3e795b27c4b
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>