drm/i915: Require object lock when freeing pages during destruction

TTM already requires this, and we require it for delayed destroy.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211216142749.1966107-11-maarten.lankhorst@linux.intel.com