drm/msm/dpu: reorder pointer operations after sanity checks to avoid NULL deref

_dpu_encoder_trigger_start dereferences "struct dpu_encoder_phys *phys"
before the sanity checks which can lead to a NULL pointer dereference if
phys is NULL.

Fix this by reordering the dereference after the sanity checks.

Fixes: 8144d17a81d9 ("drm/msm/dpu: Skip trigger flush and start for CWB")
Reviewed-by: Dmitry Baryshkov <[e-mail]>
Signed-off-by: Qasim Ijaz <[e-mail]>
Reviewed-by: Jessica Zhang <[e-mail]>
Reviewed-by: Abhinav Kumar <[e-mail]>
Patchwork: https://patchwork.freedesktop.org/patch/647536/
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Abhinav Kumar <[e-mail]>

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

···
  */
 static void _dpu_encoder_trigger_start(struct dpu_encoder_phys *phys)
 {
-	struct dpu_encoder_virt *dpu_enc = to_dpu_encoder_virt(phys->parent);
+	struct dpu_encoder_virt *dpu_enc;
 
 	if (!phys) {
 		DPU_ERROR("invalid argument(s)\n");
···
 		DPU_ERROR("invalid pingpong hw\n");
 		return;
 	}
+
+	dpu_enc = to_dpu_encoder_virt(phys->parent);
 
 	if (phys->parent->encoder_type == DRM_MODE_ENCODER_VIRTUAL &&
 	    dpu_enc->cwb_mask) {