mdss: mdp: configure secure flag for writeback block

Writeback block needs secure destination flag configured
while it is writing content protected buffer. This change
updates the flag at validate time to keep reusing the same
path for secure and non secure display.

Change-Id: Ia58276b472429731cfc23158db105a6ae0d041be
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>