drm/i915: Use hw degamma LUT for sw gamma on glk with YCbCr output

drm/i915: Use hw degamma LUT for sw gamma on glk with YCbCr output

On glk we can no longer reorder the hw LUTS vs. pipe CSC like
we could on earlier platforms, and neither do we have a
separate output CSC like on icl+. That means if we use the
pipe CSC for YCbCr output we are currently applying the gamma
LUT after the RGB->YCbCr conversion, which is just wrong.
The further we go from a linear curve the more distorted the
resulting colors become.

To work around this terrible limitation the best we can do is
repurpose the hw degamma LUT as a poor man's gamma LUT. Now
that we have the internal pre_csc_lut attachment point that
is not particularly hard to do.

What makes this less than ideal however is the fact that the
hw degamma LUT and gamma LUTs have very different capabilities.
The gamma LUT can operate in direct color type modes, whereas
the degamma LUT can't and just always operates in interpolated
mode. Additionally the degamma LUT is just a single 1D LUT, whereas
the gamma LUT is made of three separate 1D LUts (one for each channel).
So in order to make this semi-sensible we must also verify the
user supplied LUT more less matches the hw degamma LUT capabilities.
We still end up losing most of the LUT entries though, so the results
might be a bit crap.

The other option of flat out rejecting the YCbCr+gamma LUT combo
seems extremely likely to just cause a black screen for the user.
Eg. pretty sure Xorg always applies some kind of gamma LUT, and
if the user then plugs in a display that needs YCbCr output we're
toast.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221114153732.11773-18-ville.syrjala@linux.intel.com
Reviewed-by: Uma Shankar <uma.shankar@intel.com>