drm/xe: fix NPD in bo_meminfo()

Summary

In the Linux kernel, the following vulnerability has been resolved: drm/xe: fix NPD in bo_meminfo() When a buffer object is purged, its ttm.resource is set to NULL via the TTM pipeline gutting flow. However, the BO remains in the client's object list until userspace explicitly closes the GEM handle. If memory stats are queried during this time, accessing bo->ttm.resource->mem_type will result in a NULL pointer dereference. Fix this by safely skipping purged BOs in bo_meminfo, as they no longer consume any memory. User is getting NPD on device resume, and possible theory is that in bo_move(), if we need to evict something to SYSTEM to save the CCS state, but the BO is marked as dontneed, this won't trigger a move but will nuke the pages, leaving us with a NULL bo resource. And the meminfo() doesn't look ready to handle a NULL resource. v2 (Sashiko): - There could potentially be other cases where we might end up with a NULL resource, so make this a general NULL check for now. (cherry picked from commit c9a8e7daa0afe3161111e27fd92176e608c7f186)