drm/i915: move HSW+ gamma mode read to intel_color

Abstract the platform specific register access better. The separate
hsw_read_gamma_mode() will make more sense with the following changes.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b7ddcc8b0fb783eb149864070821bdb695c40366.1692287501.git.jani.nikula@intel.com