ion: msm: update msm extensions for new user handle type

The underlying type of ion_user_handle_t will be changing in the near
future. Update the MSM Ion code to use the new type rather than the old
`struct ion_handle *' so that things won't break when the migration of
the underlying type occurs.

Change-Id: Ibbfa2cc5a3aef0bd8e72d8d39054c8e59f4dab97
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>