Revert "gpu: ion: system_heap: remove superfluous cache flush"

This reverts commit 4b4056b9.

This change was causing a crash in video. Root cause analysis is
ongoing. The commit being reverted doesn't provide a very large
performance benefit anyways, so just revert it.

Change-Id: Iac1865e78c46d836ce82c0069654b05fd636094e
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>