drm/i915: i915_active.retire() is optional

Check that i915_active.retire() exists before calling.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190819075835.20065-6-chris@chris-wilson.co.uk