gpu: ion: Add EXPORT_SYMBOL to functions

Several functions in the ion interface is missing
EXPORT_SYMBOL. This is needed to allow clients to
use these functions from kernel modules.

Add EXPORT_SYMBOL to functions that are supposed
to be exposed.

Change-Id: I25611f7c367fd131daf5fb652abbea77e89a3d06
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>