msm: mdss: enable setting secure destination for writeback

New MDSS revision allows setting secure destination flag to tell
writeback block that contents should be written back to secure buffer.
This overrides previous approach to allocate a secure source pipe for
the sole purpose of enabling secure content writeback.

CRs-Fixed: 472679
Change-Id: I073db54ac930dbdead9754dec3a7dbfb0dbee316
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>