mdss: ppp: Prevent ppp from blitting req that is already completed

During initial development a mistake was made thinking only one
thread could be in workqueue handler, which was incorrect,
so ppp would process request that had already been processed. This
would result in an IOMMU page fault in some corner case.

Change-Id: I2f2484cee5d069e171ff9edec7adc6aeb35ac2b6
Signed-off-by: Terence Hampson <thampson@codeaurora.org>