drm/i915/selftests: Fix active retire callback alignment

__i915_active_call annotation is required on the retire callback to ensure
correct function alignment.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210429083530.849546-2-tvrtko.ursulin@linux.intel.com


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>