drm/xe: Check empty pinned BO list with lock held.

Use lock that is meant to use for accessing the BO pin list.

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240528115408.22094-1-nirmoy.das@intel.com


Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>