drm/i915/display: Extract ilk_read_luts()

For ilk, add hw read out to create hw blob of gamma
lut values.

v4:  -No need to initialize *blob [Jani]
     -Removed right shifts [Jani]
     -Dropped dev local var [Jani]
v5:  -Returned blob instead of assigning it internally within the
      function [Ville]
     -Renamed ilk_get_color_config() to ilk_read_luts() [Ville]
v9:  -80 character limit [Uma]
     -Made read func para as const [Ville, Uma]
     -Renamed ilk_read_gamma_lut() to ilk_read_lut_10() [Uma, Ville]
v10: -Made ilk_read_luts() static [Jani]
     -ilk_load_lut_10 has lut_size, not (lut_size - 1) [Jani]

Signed-off-by: Swati Sharma <swati2.sharma@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1567538578-4489-7-git-send-email-swati2.sharma@intel.com