GBE: Clear Flag register to fix a gpu hang.

GBE: Clear Flag register to fix a gpu hang.

When group size not aligned to simdWidth, prediction any8/16h will
calculate pmask also using flag register bits mapped to non-active
lanes. As flag register is not cleared by default, any8/16h used
for jmpi instruction may cause wrong jump, and possibly infinite loop.

So, we clear Flag register to 0 to make any8/16h prediction work correct.

Signed-off-by: Ruiling Song <ruiling.song@intel.com>
Reviewed-by: Zhigang Gong <zhigang.gong@linux.intel.com>