drm/i915: Add a simple is-bound check before unbinding

Only acquire the various atomic references required to unbind the vma if
we do need to unbind the vma.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191222210256.2066451-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -138,6 +138,10 @@ int i915_gem_object_unbind(struct drm_i915_gem_object *obj,
 		struct i915_address_space *vm = vma->vm;
 		bool awake = false;
 
+		list_move_tail(&vma->obj_link, &still_in_list);
+		if (!i915_vma_is_bound(vma, I915_VMA_BIND_MASK))
+			continue;
+
 		ret = -EAGAIN;
 		if (!i915_vm_tryopen(vm))
 			break;
@@ -153,7 +157,6 @@ int i915_gem_object_unbind(struct drm_i915_gem_object *obj,
 			}
 		}
 
-		list_move_tail(&vma->obj_link, &still_in_list);
 		spin_unlock(&obj->vma.lock);
 
 		ret = -EBUSY;