drm/radeon: only warn once in radeon_ttm_bo_destroy if va list not empty

Encountered a dozen of exact same backtraces when mesa's
pb_cache_release_all_buffers is called after that a gpu reset failed.

v2: Remove superfluous error message added in v1.

bug: https://bugs.freedesktop.org/show_bug.cgi?id=96271

Signed-off-by: Julien Isorce <jisorce@oblong.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_object.c

@@ -81,7 +81,7 @@ static void radeon_ttm_bo_destroy(struct ttm_buffer_object *tbo)
 	list_del_init(&bo->list);
 	mutex_unlock(&bo->rdev->gem.mutex);
 	radeon_bo_clear_surface_reg(bo);
-	WARN_ON(!list_empty(&bo->va));
+	WARN_ON_ONCE(!list_empty(&bo->va));
 	drm_gem_object_release(&bo->gem_base);
 	kfree(bo);
 }