kmscompositemixer: Send a flush stop event before EOS
kmscompositemixer: Send a flush stop event before EOS

Check if pad is in flushing state, and send a flush stop to
avoid the pad drops the EOS event

Change-Id: I4365a848f19d921c4126c15cbe7086bd7a0fcf53

TribeMedia/kms-elements,shelsonjava/kms-elements,TribeMedia/kms-elements,TribeMedia/kms-elements,shelsonjava/kms-elements,shelsonjava/kms-elements