drm/msm/dpu: Fix pointer dereferenced before checking

The phys_enc->wb_idx is dereferencing before null checking, so move
it after checking.

Signed-off-by: Haowen Bai <[e-mail]>
Reviewed-by: Dmitry Baryshkov <[e-mail]>
Fixes: d7d0e73f7de33 ("drm/msm/dpu: introduce the dpu_encoder_phys_* for
Reviewed-by: Abhinav Kumar <[e-mail]>
Patchwork: https://patchwork.freedesktop.org/patch/487606/
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Abhinav Kumar <[e-mail]>

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

···
  */
 static void dpu_encoder_phys_wb_destroy(struct dpu_encoder_phys *phys_enc)
 {
-	DPU_DEBUG("[wb:%d]\n", phys_enc->wb_idx - WB_0);
-
 	if (!phys_enc)
 		return;
+
+	DPU_DEBUG("[wb:%d]\n", phys_enc->wb_idx - WB_0);
 
 	kfree(phys_enc);
 }