kmsalphablending: Replace kmsalphablendingportdata by kmsgenericstructure

Change-Id: Idf06f8f1fc97903e3732283222321e59ae5c535e
kmsalphablending: Fix race condition accessing elements of the structure

Change-Id: Id1517f9ec9c7e3ef1da9d77e876dadc23ee4e92e