drm/xe: Decouple xe_exec_queue and xe_lrc

Decouple xe_lrc from xe_exec_queue and reference count xe_lrc.
Removing hard coupling between xe_exec_queue and xe_lrc allows
flexible design where the user interface xe_exec_queue can be
destroyed independent of the hardware/firmware interface xe_lrc.

v2: Fix lrc indexing in wq_item_append()

Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240530032211.29299-1-niranjana.vishwanathapura@intel.com