drm/i915/skl: Ring frequency table programming changes

Ring frequency table programming changes for SKL. No need for a
floor on ring frequency, as the issue of performance impact with
ring running below DDR frequency, is believed to be fixed on SKL

v2: Removed the check for avoiding ring frequency programming for BXT (Rodrigo)

Issue: VIZ-5144
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>