drm/i915: Set up ILK/SNB csc unit properly for YCbCr output

drm/i915: Set up ILK/SNB csc unit properly for YCbCr output

Prepare the pipe csc for YCbCr output on ilk/snb. The main difference
to IVB+ is the lack of explicit post offsets, and instead we must
configure the CSC info RGB->YUV mode (which takes care of offsetting
Cb/Cr properly) and enable the "black screen offset" bit to add the
required offset to Y.

And while at it throw some comments around the bit defines to
document which platforms have which bits.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190718145053.25808-12-ville.syrjala@linux.intel.com
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>