drm/i915: trivial: remove unneed set to NULL

At this point of the code the obj var is already NULL, so we don't
need to set it again to NULL.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>