drm/xe: improve dma-resv handling for backup object

drm/xe: improve dma-resv handling for backup object

Since the dma-resv is shared we don't need to reserve and add a fence
slot fence twice, plus no need to loop through the dependencies.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://lore.kernel.org/r/20250829164715.720735-2-matthew.auld@intel.com