drm/i915: Pull obj->userfault tracking under the ggtt->mutex

Since we want to revoke the ggtt vma from only under the ggtt->mutex, we
need to move protection of the userfault tracking from the struct_mutex
to the ggtt->mutex.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190822060914.2671-2-chris@chris-wilson.co.uk