ion: unmap ion buffer before free

Buffers must be unmapped first and then freed. Without
this sequence, specifically for secure buffers, TZ throws
error on free if it finds persisting mapping. Fix this.

Change-Id: Iba09cb998b4f87da3e9eb1c2bc186453be02a278
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>