ion: replace userspace handle cookies with idr

Userspace handles should not leak kernel virtual addresses to
userspace.  They have to be validated by looking them up in an
rbtree anyways, so replace them with an idr and validate them
by using idr_find to convert the id number to the struct
ion_handle pointer.

Signed-off-by: Colin Cross <ccross@android.com>
Change-Id: Iab5667ba6f3a73256fec3949b23a9a6f8e14a283