msm: mdss: fix writeback offset configuration in ctl path

Invalid writeback offset setting in control path
causes wrong WB output selection. This change fixes it
with correct offset configuration.

Change-Id: I9cca97d6e1241e2eba76a41d46b5fb5641fee948
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>