gpu: ion: Reference count protect/unprotect calls

Reference count the calls to ion_{un}secure_heap. The secure SCM
call will only be made when the count goes from 0 -> 1 and the
unsecure call will only be made when the count goes from 1 -> 0.

Change-Id: Ia221b79782c9223d9853ef00c2848e1ab7a73091
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>