drm/i915: Move fiddling with engine->last_retired_context

Move the knowledge about resetting the current context tracking on the
engine from inside i915_gem_context.c into intel_engine_cs.c

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180517212633.24934-2-chris@chris-wilson.co.uk