drm/i915/display: Extract glk_read_luts()

For glk, 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 glk_get_color_config() to glk_read_luts() [Ville]
    -Added degamma validation [Ville]
v9: -80 character limit [Uma]
    -Made read func para as const [Ville, Uma]

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/1567538578-4489-8-git-send-email-swati2.sharma@intel.com