msm: mdss: initialize write back arguments in kickoff

The kickoff code relies on the fact that write back arguments are zero
initialized to check for available buffer. If it's not zero initialized
it may pick dirty buffer address from stack.

Change-Id: I7309f8fe508640833a182a8ecdb2d3be1797fab7
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>