drm/i915/skl: Restrict the ring frequency table programming to SKL

Ring frequency table programming is not required on BXT. Added separate
checks to enable the programming only for SKL & skip for BXT.

v2: Removed the BXT check from gen6_update_ring_freq function

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