drm/i915: Propagate error from flushing the ring

... in order to avoid a BUG() and potential unbounded waits.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 4552600..3e78314 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1086,10 +1086,10 @@
 				struct drm_file *file_priv);
 void i915_gem_load(struct drm_device *dev);
 int i915_gem_init_object(struct drm_gem_object *obj);
-void i915_gem_flush_ring(struct drm_device *dev,
-			 struct intel_ring_buffer *ring,
-			 uint32_t invalidate_domains,
-			 uint32_t flush_domains);
+int __must_check i915_gem_flush_ring(struct drm_device *dev,
+				     struct intel_ring_buffer *ring,
+				     uint32_t invalidate_domains,
+				     uint32_t flush_domains);
 struct drm_i915_gem_object *i915_gem_alloc_object(struct drm_device *dev,
 						  size_t size);
 void i915_gem_free_object(struct drm_gem_object *obj);