drm/i915: Add PIPECONF YCbCr 4:4:4 programming for ILK-IVB

drm/i915: Add PIPECONF YCbCr 4:4:4 programming for ILK-IVB

On ILK-IVB the pipe colorspace is configured via PIPECONF
(as opposed to PIPEMISC in BDW+). Let's configure+readout
that stuff correctly.

Enabling YCbCr 4:4:4 output will now be a simple matter of
setting crtc_state->output_format appropriately in the encoder
.compute_config(). However, when we do that we must be
aware of the fact that YCbCr DP output doesn't seem to work
on ILK (resulting image is totally garbled), but on SNB+
it works fine. However HDMI YCbCr output does work correctly
even on ILK.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190718145053.25808-13-ville.syrjala@linux.intel.com
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>