drm/i915: Get rid of HAS_CORE_RING_FREQ

No need for HAS_CORE_RING_FREQ as that flag is actually the same as
.has_llc. Feedback from V. Syrjala.

Signed-off-by: Carlos Santa <carlos.santa@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>