drm/xe: allow non-contig VRAM kernel BO

If the kernel bo doesn't care about vmap(), either directly or
indirectly with save/restore then we don't need to force contig for such
buffers.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Satyanarayana K V P <satyanarayana.k.v.p@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Satyanarayana K V P <satyanarayana.k.v.p@intel.com>
Link: https://lore.kernel.org/r/20250403102440.266113-15-matthew.auld@intel.com