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: Ia3019e6519212bc3f1ab36525e2f530a32399a5c