drm/i915/ttm: ensure we unmap when purging

Purging can happen during swapping out, or directly invoked with the
madvise ioctl. In such cases this doesn't involve a ttm move, which
skips umapping the object.

v2(Thomas):
- add ttm_truncate helper, and just call into i915_ttm_move_notify() to
  handle the unmapping step

Fixes: 213d5092 ("drm/i915/ttm: Introduce a TTM i915 gem object backend")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220106174910.280616-4-matthew.auld@intel.com

drivers/gpu/drm/i915/gem/i915_gem_ttm.c

@@ -554,6 +554,20 @@ i915_ttm_resource_get_st(struct drm_i915_gem_object *obj,
 	return intel_region_ttm_resource_to_rsgt(obj->mm.region, res);
 }
 
+static int i915_ttm_truncate(struct drm_i915_gem_object *obj)
+{
+	struct ttm_buffer_object *bo = i915_gem_to_ttm(obj);
+	int err;
+
+	WARN_ON_ONCE(obj->mm.madv == I915_MADV_WILLNEED);
+
+	err = i915_ttm_move_notify(bo);
+	if (err)
+		return err;
+
+	return i915_ttm_purge(obj);
+}
+
 static void i915_ttm_swap_notify(struct ttm_buffer_object *bo)
 {
 	struct drm_i915_gem_object *obj = i915_ttm_to_gem(bo);
@@ -960,7 +974,7 @@ static const struct drm_i915_gem_object_ops i915_gem_ttm_obj_ops = {
 
 	.get_pages = i915_ttm_get_pages,
 	.put_pages = i915_ttm_put_pages,
-	.truncate = i915_ttm_purge,
+	.truncate = i915_ttm_truncate,
 	.shrink = i915_ttm_shrink,
 
 	.adjust_lru = i915_ttm_adjust_lru,