Not exactly sure what's the aim here, but the canonical nil object has
id == 0, we don't use negative object ids for anything. Plus all
object_id are valided by the object_idr, there's nothing we need to do
on top of that ENOENT check a bit further down.

Spotted while typing exhaustive igt coverage for all these
corner-cases.

Cc: Keith Packard <keithp@keithp.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190228144910.26488-2-daniel.vetter@ffwll.ch