On some stepping of SNB cpu, the first command to be parsed in BLT
command streamer should be MI_BATCHBUFFER_START otherwise the GPU
may hang.
Signed-off-by: NZou Nan hai <nanhai.zou@intel.com>
[ickle: rebased for -next]
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>