gpu: ion: move ion priv data structs and methods to ion_priv.h

Move ion priv data structs and methods to ion_priv.h.
This is needed to allow vendor spcific code dereferencing
ion data structs and accessing methods.

Change-Id: I4f863e0f4a59a80ec6b4468ca27ed7b96a78772b
Signed-off-by: Krishna Reddy <vdumpa@nvidia.com>
Reviewed-on: http://git-master/r/71111
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Aleksandr Frid <afrid@nvidia.com>