gpu: ion: Factor out common code on first alloc/last free

Currently, fmem must be transitioned on first allocation/
last free. Going forward, there may be other use cases to
call functions on first allocation/last free. Factor some of
this code out to avoid duplication.

Change-Id: I2649309318bc65aa2d0401c90e71ade4cc14c729
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>