ion: ion_system_heap: SetPagePrivate when splitting secure pages

The kgsl driver relies on checking the PagePrivate flag for secure memory.
Commit 471a5dd2 ("ion: system_heap:
Add a new allocation method") did not properly set this flag in all cases.
Fix this.

Change-Id: I4f29ee151158cd70a1617d365decc7d3fd8fe6ac
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>