disp: msm: sde: enable merge_3d_active in ctl when mode_3d is set

Add changes to set merge_3d_active in ctl datapath when
mode3d is enabled without half panel partial update.

Change-Id: I35c1f4c6045c4040cac730f23a6d9ce5f36cf5fb
Signed-off-by: Jayaprakash Madisetty <jmadiset@codeaurora.org>