drm/i915/display: Extract i965_read_luts()

For i965, add hw read out to create hw blob of gamma
lut values.

Review comments from old series:
https://patchwork.freedesktop.org/series/58039/



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 i965_get_color_config() to i965_read_lut() [Ville]
     -Renamed i965_get_gamma_config_10p6() to i965_read_gamma_lut_10p6()
      [Ville]
v9:  -Typo and 80 character limit [Uma]
     -Made read func para as const [Ville, Uma]
     -Renamed i965_read_gamma_lut_10p6() to i965_read_lut_10p6() [Ville, Uma]
v10: -Swapped ldw and udw while creating hw blob [Jani]
     -Added last index rgb lut value from PIPEGCMAX to h/w blob [Jani]

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/1568030503-26747-3-git-send-email-swati2.sharma@intel.com