gpu: ion: system_heap: remove superfluous cache flush

When we allocate pages from the page pool we flush the cache after
zero'ing the buffer. We then flush the cache again (residual from
pre-page pool days). Remove the redundant second flush.

Change-Id: I48df2777b870c33a532cf19d46660bed62d2ef59
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>