drm/xe: Take ref to VM in delayed snapshot

Kernel BO's don't take a ref to the VM, we need the VM for the
delayed snapshot, so take a ref to the VM in delayed snapshot.

v2:
 - Check for lrc_bo before taking a VM ref (CI)
 - Check lrc_bo->vm before taking / dropping a VM ref (CI)
 - Drop VM in xe_lrc_snapshot_free
v5:
 - Fix commit message wording (Johnathan)

Fixes: 47058633 ("drm/xe: Move lrc snapshot capturing to xe_lrc.c")
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240801154118.2547543-2-matthew.brost@intel.com