drm/msm/dpu: Add missing NULL pointer check for pingpong interface

It is checked almost always in dpu_encoder_phys_wb_setup_ctl(), but in a
single place the check is missing.
Also use convenient locals instead of phys_enc->* where available.

Cc: stable@vger.kernel.org
Fixes: d7d0e73 ("drm/msm/dpu: introduce the dpu_encoder_phys_* for writeback")
Signed-off-by: Nikolay Kuratov <kniv@yandex-team.ru>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/693860/
Link: https://lore.kernel.org/r/20251211093630.171014-1-kniv@yandex-team.ru
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>

Author: Nikolay Kuratov

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

@@ -247,14 +247,12 @@ static void dpu_encoder_phys_wb_setup_ctl(struct dpu_encoder_phys *phys_enc)
 		if (hw_cdm)
 			intf_cfg.cdm = hw_cdm->idx;
 
-		if (phys_enc->hw_pp->merge_3d && phys_enc->hw_pp->merge_3d->ops.setup_3d_mode)
-			phys_enc->hw_pp->merge_3d->ops.setup_3d_mode(phys_enc->hw_pp->merge_3d,
-					mode_3d);
+		if (hw_pp && hw_pp->merge_3d && hw_pp->merge_3d->ops.setup_3d_mode)
+			hw_pp->merge_3d->ops.setup_3d_mode(hw_pp->merge_3d, mode_3d);
 
 		/* setup which pp blk will connect to this wb */
-		if (hw_pp && phys_enc->hw_wb->ops.bind_pingpong_blk)
-			phys_enc->hw_wb->ops.bind_pingpong_blk(phys_enc->hw_wb,
-					phys_enc->hw_pp->idx);
+		if (hw_pp && hw_wb->ops.bind_pingpong_blk)
+			hw_wb->ops.bind_pingpong_blk(hw_wb, hw_pp->idx);
 
 		phys_enc->hw_ctl->ops.setup_intf_cfg(phys_enc->hw_ctl, &intf_cfg);
 	} else if (phys_enc->hw_ctl && phys_enc->hw_ctl->ops.setup_intf_cfg) {