ion: use vm_insert_pfn for faulted pages

Most ion userspace mappings are created with remap_pfn_range.  Use
vm_insert_pfn instead of vm_insert_page to make faulted cached
mappings look more like uncached mappings.

Change-Id: I9ec5cad3fef54f3b80be8b306d7ff2f1fe3f0e66
Signed-off-by: Colin Cross <ccross@android.com>