mdss: rotator: fix memory scribble issue

Prior to this patch it was possible for rotator wq to write
to memory that had been freed in close session. Close
session now offloads the close work to wq if it still has not
completed the work.

Change-Id: I4b48071f930b912a098094d7de2b2081dac4c2e5
Signed-off-by: Terence Hampson <thampson@codeaurora.org>