drm/xe/multi_queue: Protect priority against concurrent access

drm/xe/multi_queue: Protect priority against concurrent access

Use a spinlock to protect multi-queue priority being concurrently
updated by multiple set_priority ioctls and to protect against
concurrent read and write to this field.

v2: Update documentation, remove WRITE/READ_LOCK() (Thomas)
    Use scoped_guard, reduced lock scope (Matt Brost)
v3: Fix author (checkpatch)

Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patch.msgid.link/20260126174241.3470390-2-niranjana.vishwanathapura@intel.com