drm/i915: Enable SandyBridge blitter ring

Based on an original patch by Zhenyu Wang, this initializes the BLT ring for
SandyBridge and enables support for user execbuffers.

Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
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 5b37ff3..9e81ff3 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -22,6 +22,7 @@
 	enum intel_ring_id {
 		RING_RENDER = 0x1,
 		RING_BSD = 0x2,
+		RING_BLT = 0x4,
 	} id;
 	u32		mmio_base;
 	unsigned long	size;
@@ -124,6 +125,7 @@
 
 int intel_init_render_ring_buffer(struct drm_device *dev);
 int intel_init_bsd_ring_buffer(struct drm_device *dev);
+int intel_init_blt_ring_buffer(struct drm_device *dev);
 
 u32 intel_ring_get_active_head(struct drm_device *dev,
 			       struct intel_ring_buffer *ring);