gpu: ion: fix zero'ing of cached buffers in system heap

Currently, in the system heap we zero out buffers at free-time when they
came from uncached allocations. The reason for this is that uncached
allocations come from the page pools, which require this free-time
zero'ing. However, the assumption that a buffer came from the page pools
iff it was an uncached allocation is no longer valid since [0204b382
"gpu: ion: add page pooling to cached allocations from the system heap"]
introduced page pooling for cached allocations as well. This results in
us not zero'ing out cached buffers before putting them back in the page
pools, which is a security bug (we could potentially leak sensitive
information).

Fix this by zero'ing out all buffers that are going back into the page
pools (cached and uncached).

Change-Id: I5c51eca75c8cc3b3823df1494bb0eef295c77b86
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>