drm/i915: add missing unregister_oom_notifier to the error/unload path

drm/i915: add missing unregister_oom_notifier to the error/unload path

I'm trying to reduce the WARNs during driver reload and this was one of
them. Also while at it remove the redundant condition from before
unregister_shrinker().

v2:
- fix the error path too and move the unregister to its logical place
(Chris)
- remove redundant condition from before unregister_shrinker()

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>