drm/i915/userptr: Convert to drm_i915_private

drm/i915/userptr: Convert to drm_i915_private

userptr directly only uses drm_device in a single interface where it
meant to use drm_i915_private (everywhere else we have to derive it from
the drm_i915_gem_object and so require going from drm_device).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463671036-3235-1-git-send-email-chris@chris-wilson.co.uk