gpu: ion: Limit scope of local functions

Several functions that should not be used outside of
the ion.c file does not have static keyword. In
addition two of the functions should not be exported.

Add static keyword to functions that are local to the
translation unit and remove EXPORT_SYMBOL declarations
that are not needed.

Change-Id: Ia1ef54e3670d361c9a6d684f789be2129facb04d
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>