drm/i915: SNB BLT workaround

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.

(cherry picked from commit 8d19215b)

Conflicts:

	drivers/gpu/drm/i915/intel_ringbuffer.c
	drivers/gpu/drm/i915/intel_ringbuffer.h

Signed-off-by: Zou Nan hai <nanhai.zou@intel.com>
Cc: stable@kernel.org
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>