drm/i915: Rework legacy LUT handling

Currently crtc_state_is_legacy_gamma() has a very specific set
of conditions, not all of which are actually necessary. Also
when we detect those conditions check_luts() just skips all
the checks. That will no longer work for glk soon when we'll
start to use the hw degamma LUT in place of the hw gamma LUT
for YCbCr output. So let's rework the logic to only really consider
whether the user provided gamma_lut is one that matches the hw
legacy LUT capabilities or not.

We'll need to reject C8+degamma on ivb+ since the presence of
degamma_lut would either mean we have to really use the LUT
for degamma as opposed to C8 palette, or we have to enable
split gamma mode which also can't work as the C8 palette.

Otherwise this will now cause the legacy LUT to go through the
regular lut checks as well. As a side effect we also start to
allow the use of the legacy LUT with CTM, but that is perfectly
fine as far a the hardware is concerned.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221114153732.11773-17-ville.syrjala@linux.intel.com
Reviewed-by: Uma Shankar <uma.shankar@intel.com>