drm/i915: Make i915_gem_retire_requests_ring() static

Its last usage outside of i915_gem.c was removed in:

  commit 1f70999f
  Author: Chris Wilson <chris@chris-wilson.co.uk>
  Date:   Mon Jan 27 22:43:07 2014 +0000

     drm/i915: Prevent recursion by retiring requests when the ring is full
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.h

@@ -2178,7 +2178,6 @@ struct drm_i915_gem_request *
 i915_gem_find_active_request(struct intel_ring_buffer *ring);
 
 bool i915_gem_retire_requests(struct drm_device *dev);
-void i915_gem_retire_requests_ring(struct intel_ring_buffer *ring);
 int __must_check i915_gem_check_wedge(struct i915_gpu_error *error,
 				      bool interruptible);
 static inline bool i915_reset_in_progress(struct i915_gpu_error *error)

drivers/gpu/drm/i915/i915_gem.c

@@ -61,6 +61,7 @@ static unsigned long i915_gem_inactive_scan(struct shrinker *shrinker,
 static unsigned long i915_gem_purge(struct drm_i915_private *dev_priv, long target);
 static unsigned long i915_gem_shrink_all(struct drm_i915_private *dev_priv);
 static void i915_gem_object_truncate(struct drm_i915_gem_object *obj);
+static void i915_gem_retire_requests_ring(struct intel_ring_buffer *ring);
 
 static bool cpu_cache_is_coherent(struct drm_device *dev,
 				  enum i915_cache_level level)
@@ -2414,7 +2415,7 @@ void i915_gem_reset(struct drm_device *dev)
 /**
  * This function clears the request list as sequence numbers are passed.
  */
-void
+static void
 i915_gem_retire_requests_ring(struct intel_ring_buffer *ring)
 {
 	uint32_t seqno;