drm/i915: Added BXT check in HAS_CORE_RING_FREQ macro

Updated the HAS_CORE_RING_FREQ macro to add the broxton check,
so as to disallow the programming & read of ring frequency
table for it.

Issue: VIZ-5144
Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Akash Goel <akash.goel@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>