drm/i915: Drop struct_mutex from around GEM initialisation

We no longer need to placate lockdep by holding struct_mutex for our
initialisation, so don't.

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/20191004134015.13204-21-chris@chris-wilson.co.uk