drm/msm/dpu: don't always program merge_3d block

Only program the merge_3d block for the video phys encoder when the 3d
blend mode is not NONE

Fixes: 3e79527a ("drm/msm/dpu: enable merge_3d support on sm8150/sm8250")
Suggested-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/619095/
Link: https://lore.kernel.org/r/20241009-merge3d-fix-v1-1-0d0b6f5c244e@quicinc.com


Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>