[Beignet] clEnqueueNDRangeKernel and kernel completion

[Zou, Nanhai]

>> Which method do you prefer?
 
Should remove the following
If (batch->buffer != NULL) {
	dri_bo_unreference(batch->buffer);
	batch->buffer = NULL);
}
That code is duplicated.

And 
dri_bo_unreference looks more consistant.

Thanks
Zou Nanhai