drm/i915: Rename pipe CSC to use ilk_ prefix

The pipe CSC was introduced by ILK, so change everything related
to use ilk_ as the prefix.

Cc: Johnson Lin <johnson.lin@intel.com>
Cc: Uma Shankar <uma.shankar@intel.com>
Cc: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180222214232.6064-3-ville.syrjala@linux.intel.com


Reviewed-by: Uma Shankar <uma.shankar@intel.com>