ion: fix page pool cache policy

Fix redundant "buffer->private_flags & ION_PRIV_FLAG_SHRINKER_FREE"
checks in if(!cached ...) condition block.

AOSP Change-Id: I98ee8902df0c80135dddfa998c4ca4c2bb44e40e,
"ion: Handle the memory mapping correctly on x86", is broken
on android-3.18+ kernels. It conflicts with upstream commit
53a91c68, "staging: ion: Add private buffer flag to skip
page pooling on free", and break the ION_PRIV_FLAG_SHRINKER_FREE
private flag check logic.

Change-Id: I9cee4bcc3545cf92e07c21c2b42d27cf88da3316
Reported-by: chenfeng <puck.chen@hisilicon.com>
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>