msm: mdss: Set correct flush bit for writeback mixer.

Wrong flush bit is being set for writeback mixer which can cause
MDP hang/unexpected behaviour when the mixer is used in line mode.
This patch addresses the issue by setting the correct flush bit.

Change-Id: Ie0c2ee27b1f466164a8fb84f7daae9309b74d1d8
Signed-off-by: Vishnuvardhan Prodduturi <vproddut@codeaurora.org>