drm/i915/gem: Fix gem_madvise for ttm+shmem objects

Gem-TTM objects that are backed by shmem might have populated
page-vectors without having the GEM pages set. Those objects
aren't moved to the correct shrinker / purge list by gem_madvise.

For such objects, identified by having the
_SELF_MANAGED_SHRINK_LIST set, make sure they end up on the
correct list.

v2:
- Revert a change that made swapped-out objects inaccessible for
  truncating. (Matthew Auld)
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211108123637.929617-1-thomas.hellstrom@linux.intel.com

drivers/gpu/drm/i915/i915_gem.c

@@ -1005,7 +1005,8 @@ i915_gem_madvise_ioctl(struct drm_device *dev, void *data,
 			obj->ops->adjust_lru(obj);
 	}
 
-	if (i915_gem_object_has_pages(obj)) {
+	if (i915_gem_object_has_pages(obj) ||
+	    i915_gem_object_has_self_managed_shrink_list(obj)) {
 		unsigned long flags;
 
 		spin_lock_irqsave(&i915->mm.obj_lock, flags);