drm/i915: Remove whitelist application from ringbuffer backend

There is no white-listing before Gen8 and after the removal ringbuffer
support for these platforms we can remove the call to this no-op.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20181129134128.7994-1-tvrtko.ursulin@linux.intel.com