drm/i915/guc: Don't take struct_mutex for object unreference

We no longer need to take the struct_mutex for freeing objects, and on
the finalisation paths here the mutex is not been used for serialisation
of the pointer access, so remove the BKL wart.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170214133420.7977-1-chris@chris-wilson.co.uk


Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>