drm/i915: add set_tail hook in struct intel_ring_buffer

This is prepared for video codec ring buffer on Sandybridge. It is
needed to read/write more than one register to move the tail pointer of
the video codec ring on Sandybridge.

Signed-off-by: Xiang, Haihao <haihao.xiang@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 5603d6e..7bd571c 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -48,6 +48,8 @@
 			struct intel_ring_buffer *ring);
 	unsigned int	(*get_tail)(struct drm_device *dev,
 			struct intel_ring_buffer *ring);
+	void		(*set_tail)(struct drm_device *dev,
+			u32 value);
 	unsigned int	(*get_active_head)(struct drm_device *dev,
 			struct intel_ring_buffer *ring);
 	void		(*advance_ring)(struct drm_device *dev,