msm: mdss: Adjust writeback mixer offset

With new targets, number of interface layer mixers have increased.
This increases existing macro values. Even though writeback mixers
have not changed, their programming needs to be adjusted.

Change-Id: I71bb6e6988d4e2bf2f66e0ecbb7ff9672d21b8dc
Signed-off-by: Shalabh Jain <shalabhj@codeaurora.org>