drm/xe: Remove unwanted mutex locking

Do not hold xef->exec_queue.lock mutex while parsing the xarray
xef->exec_queue.xa in xe_file_close() as it is not needed and
will cause an unwanted dependency between this lock and the vm->lock.

This lock protects the exec queue lookup and reference taking which
doesn't apply to this code path. When FD is closing, IOCTLs presumably
can't be modifying the xarray.

v2: Update commit text (Matt Brost)
v3: Add more code comment (Rodrigo Vivi)
v4: Further expand code comment (Rodirgo Vivi)

Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Jagmeet Randhawa <jagmeet.randhawa@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240529221639.23117-1-niranjana.vishwanathapura@intel.com