ion: allow cached mappings of chunk and system heap buffers

Now that ion_vm_fault uses vm_insert_pfn instead of vm_insert_page
cached buffers can be supported in any heap.  Remove the checks
in the chunk and system heaps.

Change-Id: I371a44c400ed8a342c3b0eed90d0fb7060537697
Signed-off-by: Colin Cross <ccross@android.com>