drm/i915/icl: NV12 y-plane ddb is not in same plane

We don't have planar pixel format support implemented for ICL yet.
ICL require 2 display planes to be allocated for Planar formats unlike
previous GEN. So ICL/GEN11 doesn't require to write Y-plane ddb data in
NV12_BUF_CFG register and PLANE_NV12_BUF_CFG register is removed in ICL.

This patch removes the PLANE_NV12_BUF_CFG write for ICL.

Changes Since V1:
 - Improve commit message as per Paulo's comment

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: James Ausmus <james.ausmus@intel.com>
Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180130134918.32283-5-paulo.r.zanoni@intel.com