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.

Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>