drm/i915: Enable refcount debugging for default debug levels

refcount_t is our first line of defence against use-after-free, so let's
enable it for debugging.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190613122842.4840-1-chris@chris-wilson.co.uk