video: rockchip: mpp: control the log, when task is null

tips:
    in rk3328, avsd and vdpu share the same interrupt number,
    so, when each one hardware done, avsd and vdpu irq will response.
    this is a normal log when meet the case.

Change-Id: I9fea220a64d7310805194d9f71c24db0cef1491e
Signed-off-by: Ding Wei <leo.ding@rock-chips.com>

drivers/video/rockchip/mpp/mpp_common.c

@@ -1751,7 +1751,7 @@ irqreturn_t mpp_dev_irq(int irq, void *param)
 			set_bit(TASK_STATE_IRQ, &task->state);
 		}
 	} else {
-		mpp_err("error, task is null\n");
+		mpp_debug(DEBUG_IRQ_CHECK, "error, task is null\n");
 	}
 done:
 	return irq_ret;