drm/i915/icl: implement DVFS for ICL

ICL DVFS is almost the same as CNL, except for the CDCLK/DDICLK
table. Implement it just like CNL does.

References: commit 48469ece ("drm/i915: Use cdclk_state->voltage
 on CNL")
References: commit 53e9bf5e

 ("drm/i915: Adjust system agent
 voltage on CNL if required by DDI ports")
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180614221018.19044-1-paulo.r.zanoni@intel.com