drm/msm/sde: Fix roi incorrect offset of ad4 core1 in the merge mode

The patch moves the roi area of AD4 core1 by 6 pixels to the right
in the merge mode.

Change-Id: Ie054b2fd225f98720b517fe1e16f48fec24a1476
Signed-off-by: Yuchao Ma <yuchaom@codeaurora.org>