[media] mtk-vcodec: fix compiler warning

mtk-vcodec/venc_vpu_if.c:40:30: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
  struct venc_vpu_inst *vpu = (struct venc_vpu_inst *)msg->venc_inst;
                              ^

Note: venc_inst is u64.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/platform/mtk-vcodec/venc_vpu_if.c

@@ -37,7 +37,8 @@ static void handle_enc_encode_msg(struct venc_vpu_inst *vpu, void *data)
 static void vpu_enc_ipi_handler(void *data, unsigned int len, void *priv)
 {
 	struct venc_vpu_ipi_msg_common *msg = data;
-	struct venc_vpu_inst *vpu = (struct venc_vpu_inst *)msg->venc_inst;
+	struct venc_vpu_inst *vpu =
+		(struct venc_vpu_inst *)(unsigned long)msg->venc_inst;
 
 	mtk_vcodec_debug(vpu, "msg_id %x inst %p status %d",
 			 msg->msg_id, vpu, msg->status);