gpu: ion: ion_chunk_heap: Zero chunk heap memory at creation time

Allocations from the ion heap need to be zeroed to protect userspace
from seeing memory belonging to other processes.  First allocations
from this heap were not zero'd allowing users to see memory from other
processes on a warm reset.

Change-Id: I524a7b79cb76c390c870fcf8b30d213185fc85a0
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>