drm/i915: Add i915_sched_engine destroy vfunc

This is required to allow backend specific cleanup

v2:
 (John H)
  - Rework commit message

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210727002348.97202-7-matthew.brost@intel.com