drm/i915: Fixup userptr mmu notifier registration error handling

Avoid dereferencing the error pointer and also avoid returning NULL
from i915_mmu_notifier_find since the callers do not expect that.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 7741b547 ("drm/i915: Preallocate our mmu notifier workequeu to unbreak cpu hotplug deadlock")
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171017150908.12840-1-tvrtko.ursulin@linux.intel.com