ion: Correct argument of Set/ClearPagePrivate of secure CMA

On each sg of secure CMA buffer to set/clear page private bit,
we should use the sg_page(sg) rather than sg_page(sgt->sgl).

Change-Id: I05300d9d3a0931d2a0c113f945b45136e0dea01e
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>