drm/i915: Do not unlock upon error in i915_gem_idle()

We never took the lock ourselves and all callers expect the struct_mutex
to be locked upon return (be it success or error), thereore dropping the
lock along the error paths looks to be a vestigial error from

commit db1b76ca6a79c774074ae87bee7afc0825a478f5
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Tue Jul 9 16:51:37 2013 +0200

    drm/i915: don't frob mm.suspended when not using ums

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>