drm/i915: do not export the instances of struct intel_ring_buffer

Introduce intel_init_render_ring_buffer(), intel_init_bsd_ring_buffer
for ring initialization.

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 8dc0e62..5603d6e 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -133,7 +133,7 @@
 u32 intel_ring_get_seqno(struct drm_device *dev,
 		struct intel_ring_buffer *ring);
 
-extern struct intel_ring_buffer render_ring;
-extern struct intel_ring_buffer bsd_ring;
+int intel_init_render_ring_buffer(struct drm_device *dev);
+int intel_init_bsd_ring_buffer(struct drm_device *dev);
 
 #endif /* _INTEL_RINGBUFFER_H_ */