drm/xe: Fix exec queue usage for unbinds

Passing in a NULL exec queue to __xe_pt_unbind_vma results in the
migrate exec queue being used. This is not the intent from the VM bind
IOCTL, rather a NULL exec queue should use default VM exec queue.

Reviewed-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>