gpu: ion: Switch to using a single shrink function

The single shrink function will free lower order pages first. This
enables compaction to work properly.

Change-Id: Iddc5b2afbaec1d122de303c42a549f8f5d16c6cf
Git-commit: 38de9828c544c985d1d4a7c2167efd94222f3bf1
Git-repo: https://android.googlesource.com/kernel/common


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