drm/xe/vm: Don't pin the vm_resv during validation

The pinning has the odd side-effect that unlocking *any* resv
during validation triggers an "unlocking pinned lock" warning.

Cc: Matthew Brost <matthew.brost@intel.com>
Fixes: 9d555864 ("drm/xe: Rework eviction rejection of bound external bos")
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250821143045.106005-2-thomas.hellstrom@linux.intel.com