gpu: ion: system_heap: invalidate cache on cached pool allocations

We need to invalidate the cache when allocating pages for the cached
page pools to avoid the possibility of a stale cached value from a
different mapping being written back after we allocate our page.

Change-Id: I9b0ebfd143850cf4a4ef309a264f947a57abff77
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>