gpu: ion: Add chunk heap

This patch adds support for a chunk heap that allows for buffers that are
made up of a list of fixed size chunks taken from a carveout.  Chunk sizes
are configured when the heaps are created by passing the chunk size in the
priv field of the heap platform data.

Change-Id: Ia9e003f727b553a92804264debe119dcf78b14e0
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>