ion: export a few handle-related APIs

There are a few APIs (ion_handle_put and ion_handle_get_by_id) that are
useful for implementing Ion extensions (e.g. custom ioctl
handlers). Remove the `static' to make these APIs available to other Ion
code.

Change-Id: Ice8767853f628d8178e96a36db3da02fa97f3235
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>