overlay: Reset display and mixer tags for unallocated pipes

When clients request pipes and unused pipe-slots are returned (the
ones having DPY_UNUSED, MIXER_UNUSED), they are tagged with requested
display and mixer ids.

If the client releases such pipes without using them (i.e without a
successful driver commit) we need to reset display and mixer tags to
UNUSED, so subsequent requests for a different display/mixer succeed.

Change-Id: I26e9fc4da3c3919ee23b65e2053c762df6c4cf8c
1 file changed