staging: android: ion: Use CMA APIs directly

When CMA was first introduced, its primary use was for DMA allocation
and the only way to get CMA memory was to call dma_alloc_coherent. This
put Ion in an awkward position since there was no device structure
readily available and setting one up messed up the coherency model.
These days, CMA can be allocated directly from the APIs. Switch to using
this model to avoid needing a dummy device. This also mitigates some of
the caching problems (e.g. dma_alloc_coherent only returning uncached
memory).

Signed-off-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>