liboverlay : unset basepipe on overlay intialization.

On display framework intialization or restart when we are unsetting
stale MDP pipes connected to mixer, free base pipe too. Base pipe
gets allocated before composition once on start or restart of framework.

Change-Id: Ie73915031ffbcf03b6cadf2c526c76be79d582be
CRs-fixed: 483142
Merge remote-tracking branch 'cm/cm-10.1' into jb-mr1