drm/i915/gem: Rework error handling in default_engines

Since free_engines works for partially constructed engine sets, we can
use the usual goto pattern.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708154835.528166-18-jason@jlekstrand.net