sdm: Unregister fb_ids after Validate

HWC allows destruction of layers between Validate and Commit.
Buffer fds could also get updated in between. Unregister fb_id after
Validate to avoid carrying over stale fds as keys to Commit

Change-Id: Ic26a7644ffb1fb4da26b24364a33ac9293031867
CRs-Fixed: 2157977
3 files changed