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 8d19215be8)

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>

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -63,6 +63,7 @@ struct  intel_ring_buffer {
 			struct drm_i915_gem_execbuffer2 *exec,
 			struct drm_clip_rect *cliprects,
 			uint64_t exec_offset);
+	void		(*cleanup)(struct intel_ring_buffer *ring);
 
 	/**
 	 * List of objects currently involved in rendering from the
@@ -98,6 +99,8 @@ struct  intel_ring_buffer {
 
 	wait_queue_head_t irq_queue;
 	drm_local_map_t map;
+
+	void *private;
 };
 
 static inline u32