drm/i915/buddy: add missing call to i915_global_register

We are meant to register the kmem cache at init, such the supplied exit
and shrink hooks can be called.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190905072921.7979-1-matthew.auld@intel.com