drm/i915: Rename dev_priv->ring to dev_priv->render_ring.

With the advent of the BSD ring, be clear about which ring this is.
The docs are pretty consistent with calling this the Render engine at
this point.

drivers/gpu/drm/i915/i915_drv.c

@@ -389,7 +389,7 @@ int i965_reset(struct drm_device *dev, u8 flags)
 	 */
 	if (drm_core_check_feature(dev, DRIVER_MODESET) ||
 	    !dev_priv->mm.suspended) {
-		drm_i915_ring_buffer_t *ring = &dev_priv->ring;
+		drm_i915_ring_buffer_t *ring = &dev_priv->render_ring;
 		struct drm_gem_object *obj = ring->ring_obj;
 		struct drm_i915_gem_object *obj_priv = to_intel_bo(obj);
 		dev_priv->mm.suspended = 0;