ion: Drop cache operation after zeroing

At one time, the zeroing algorithm used a mixture of
cached and uncached mappings which necessitated an extra
cache operation afterwards. The zeroing algorithm has been
simplified so the cache operation is no longer required if
the mapping used for zeroing matches the cache status of
the buffer (cached or not). Remove the cache operation and
use appropriate page protections instead.

Change-Id: I7d3596ed503811818782e77a0f3f7f63b1456d70
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>