drm/i915/display: Extract chv_read_luts()

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

Review comments from previous series:
https://patchwork.freedesktop.org/patch/328252



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 function cherryview_get_color_config() to chv_read_luts()
    -Renamed cherryview_get_gamma_config() to chv_read_cgm_gamma_lut()
     [Ville]
v9: -80 character limit [Uma]
    -Made read func para as const [Ville, Uma]
    -Renamed chv_read_cgm_gamma_lut() to chv_read_cgm_gamma_lut()
     [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/1568030503-26747-4-git-send-email-swati2.sharma@intel.com