ion: remove ion_heap_alloc_pages

Now that ion_vm_fault doesn't need a struct page with a nonzero
refcount, there is no need allocate heap memory for cached pages using
split_page.  Remove the ion_heap_alloc_pages and ion_heap_free_pages
helpers in favor of direct calls to alloc_pages and __free_pages,
and remove the special handling in the system heap.

Change-Id: I5966a798f48df2d56642e662a69c1495944f6509
Signed-off-by: Colin Cross <ccross@android.com>