drm/xe: share bo dma-resv with backup object

We end up needing to grab both locks together anyway and keep them held
until we complete the copy or add the fence. Plus the backup_obj is
short lived and tied to the parent object, so seems reasonable to share
the same dma-resv. This will simplify the locking here, and in follow
up patches.

v2:
  - Hold reference to the parent bo to be sure the shared dma-resv can't
    go out of scope too soon. (Thomas)

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://lore.kernel.org/r/20250416150913.434369-7-matthew.auld@intel.com