drm/i915: Wrap drm_gem_object_lookup in i915_gem_object_lookup

For symmetry with a forthcoming i915_gem_object_get() and
i915_gem_object_put().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1469005202-9659-4-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469017917-15134-3-git-send-email-chris@chris-wilson.co.uk