drm/msm/sde: Update dither programming for dual-pipe merge modes

For DUALPIPE_DSCMERGE, DUALPIPE_3DMERGE and DUALPIPE_3DMERGE_DSC
topologies, dither needs to be programmed on all ping-pong
buffers used in dual pipe case.

Change-Id: I9894a722a52e1b3182cc402db3796a173a07a3c2
Signed-off-by: Ping Li <pingli@codeaurora.org>