drm/i915: Add HAS_CORE_RING_FREQ macro

Added a new HAS_CORE_RING_FREQ macro, currently used in
gen6_update_ring_freq & i915_ring_freq_table debugfs function.
The programming & read of ring frequency table is needed for newer
GEN(>=6) platforms, except VLV/CHV.

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