kmsalphablending: EOS callback is not blocking so we can return GST_PAD_PROBE_OK value

Change-Id: I994f3d34fed3833e6c18a194189935b63c57d7da
kmsalphablending: Fix memory leak produced when EOS is processed before the destroy port function

Change-Id: Ifc39985490c46e3154d32a12151711ba53b74acf