drm/i915: Update i915_scheduler to operate on i915_sched_engine

Rather passing around an intel_engine_cs in the scheduling code, pass
around a i915_sched_engine.

v3:
 (Jason Ekstrand)
  Add READ_ONCE around rq->engine in lock_sched_engine

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210618010638.98941-8-matthew.brost@intel.com