drm/i915: Introduce vfunc read_luts() to create hw lut

drm/i915: Introduce vfunc read_luts() to create hw lut

In this patch, a vfunc read_luts() is introduced to create a hw lut
i.e. lut having values read from gamma/degamma registers which will
later be used to compare with sw lut to validate gamma/degamma lut values.

v3: -Rebase
v4: -Renamed intel_get_color_config to intel_color_get_config [Jani]
    -Wrapped get_color_config() [Jani]
v5: -Renamed intel_color_get_config() to intel_color_read_luts()
    -Renamed get_color_config to read_luts
v6: -Renamed intel_color_read_luts() back to intel_color_get_config()
     [Jani and Ville]

Signed-off-by: Swati Sharma <swati2.sharma@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1559123462-7343-2-git-send-email-swati2.sharma@intel.com