drm: Fixup locking WARN_ON mistake around gem_object_free_unlocked

Embarrassingly while fixing up the old paths for i915 I managed to
misplace a locking check for the new _unlocked paths. That's what I
get for not retesting on radeon.

Fixes: 9f0ba539

 ("drm/gem: support BO freeing without dev->struct_mutex")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Alex Deucher <alexdeucher@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>