drm/i915: Enable intel_color_get_config()

In this patch, intel_color_get_config() is enabled and support
for read_luts() will be added platform by platform incrementally
in the follow-up patches.

v4: -Renamed intel_get_color_config to intel_color_get_config [Jani]
    -Added the user early on such that support for get_color_config()
     can be added platform by platform incrementally [Jani]
v5: -Incorrect place for calling intel_color_get_config() in
     haswell_get_pipe_config() [Ville]
v6: -Renamed intel_color_read_luts() 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/1559123462-7343-3-git-send-email-swati2.sharma@intel.com