gpu: ion: optimize system heap for non fault buffers

If a buffer's user mappings are not going to be faulted
in it need not be allocated page wise.  We can optimize
this common case by allocating an sglist of larger chunks
rather than creating an entry for each page in the
allocation.

Change-Id: I47814990e55c7bdb7abeaa2af824744b0a97602d
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>