drm/i915/color: Extract icl_read_luts()

For icl+, have hw read out to create hw blob of gamma
lut values. icl+ platforms supports multi segmented gamma
mode by default, add hw lut creation for this mode.

This will be used to validate gamma programming using dsb
(display state buffer) which is a tgl specific feature.

v2: -readout code for multisegmented gamma has to come
     up with some intermediate entries that aren't preserved
     in hardware (Jani N)
    -linear interpolation (Ville)
    -moved common code to check gamma_enable to specific funcs,
     since icl doesn't support that
v3: -use u16 instead of __u16 [Jani N]
    -used single lut [Jani N]
    -improved and more readable for loops [Jani N]
    -read values directly to actual locations and then fill gaps [Jani N]
    -moved cleaning to patch 1 [Jani N]
    -renamed icl_read_lut_multi_seg() to icl_read_lut_multi_segment to
     make it similar to icl_load_luts()
    -renamed icl_compute_interpolated_gamma_blob() to
     icl_compute_interpolated_gamma_lut_values() more sensible, I guess
v4: -removed interpolated func for creating gamma lut values
    -removed readouts of fine and coarse segments, failure to read PAL_PREC_DATA
     correctly
v5: -added gamma_enable check inside read_luts()
v6: -renamed intel_color_lut_entry_equal() to intel_color_lut_entries_equal() [Ville]
    -changed if-else to switch [Ville]
    -removed intel_color_lut_entry_multi_equal() [Ville]
v7: -checkpatch warnings
v8: -rebased
v9: -rebased, aligned with Ville's style of gamma cleanup

Signed-off-by: Swati Sharma <swati2.sharma@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200317135736.14305-1-swati2.sharma@intel.com