drm/i915: Pull kref into i915_address_space

Make the kref common to both derived structs (i915_ggtt and i915_ppgtt)
so that we can safely reference count an abstract ctx->vm address space.

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/20190611091238.15808-1-chris@chris-wilson.co.uk