drm/xe: Make exec_queue_kill safe to call twice

An upcoming PXP patch will kill queues at runtime when a PXP
invalidation event occurs, so we need exec_queue_kill to be safe to call
multiple times.

v2: Add documentation (Matt B)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240814205654.1716586-1-daniele.ceraolospurio@intel.com