[Beignet] [PATCH] Add memory fence before barrier to support global memory barrier.

[Zou, Nanhai]

>>IMO, it will not only dispatch one thread gropu at one time. The GPGPU walker will dispatch as much as possible thread group at one time based on current available EUs. The hardware just need to ensure one work/thread group should not exceed one half-slice's boundary.

I think we should limit the thread group size according to SLM size per half slice. HW will not automatically split them.

Thanks
Zou Nanhai.