mdss: overlay: Don't call kick-off for writeback panel on open

Do not call kick off on open for writeback panels, since at beginning of
writeback there are no output buffers queued

Change-Id: Iaf9b9abd03ff22b13e6e601af12549c5a15957f1
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>
Signed-off-by: Saurabh Shah <saurshah@codeaurora.org>