drm/i915: Remove bogus NV12 PLANE_COLOR_CTL setup

drm/i915: Remove bogus NV12 PLANE_COLOR_CTL setup

We already handle the color encoding mode properly. Remove the broken
NV12 special case.

Cc: Vidya Srinivas <vidya.srinivas@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Fixes: 8ed30ab6aced ("drm/i915: Enable YUV to RGB for Gen10 in Plane Ctrl Reg")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180521185613.5097-1-ville.syrjala@linux.intel.com
Reviewed-By: Vidya Srinivas <vidya.srinivas@intel.com>