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: 3e79527 ("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>

Author: Jessica Zhang

drivers/gpu/drm/msm/disp/dpu1/dpu_encoder_phys_vid.c

@@ -302,7 +302,7 @@ static void dpu_encoder_phys_vid_setup_timing_engine(
 	intf_cfg.stream_sel = 0; /* Don't care value for video mode */
 	intf_cfg.mode_3d = dpu_encoder_helper_get_3d_blend_mode(phys_enc);
 	intf_cfg.dsc = dpu_encoder_helper_get_dsc(phys_enc);
-	if (phys_enc->hw_pp->merge_3d)
+	if (intf_cfg.mode_3d && phys_enc->hw_pp->merge_3d)
 		intf_cfg.merge_3d = phys_enc->hw_pp->merge_3d->idx;
 
 	spin_lock_irqsave(phys_enc->enc_spinlock, lock_flags);