ion: ensure CMO target is valid

Cleanup ION cache maintenance code to properly validate the target of
userspace cache maintenance requests.

Bug: 72957321
Change-Id: I55b8e3584c59634f95250bc7c0bce5d8d70e6a13
Signed-off-by: Liam Mark <lmark@codeaurora.org>
Signed-off-by: Swetha Chikkaboraiah <schikk@codeaurora.org>