drm/msm/sde: add support to configure 3d merge block

3d merge programming has been modified in sdm855. This change adds
support to program the merge_3d block. The merge 3d block is tied to
the pingpong buffer selected, this change adds additional support in
pingpong block to program the 3d merged block.

Change-Id: I9bc015ce4694bb0fa89881b1f955852cdee973c1
Signed-off-by: Steve Cohen <cohens@codeaurora.org>
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>