ion: Assume in-kernel clients for prefetch and drain

The current implementation of prefetch and drain for the
system secure heap assumes that all of the buffers that it
handles are userspace buffers. Since prefetch and drain will
no longer be directly invoked from userspace, assume that
callers will provide any information needed for prefetch/drain
in buffers that are allocated within the kernel, and now that
they are ready for use, wire up the prefetch and drain
implementations for the system secure heap.

Change-Id: Ice22b8da39188dff9e1c5253a167437349d0f2d0
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>