drm/i915/regs: split out intel_color_regs.h

Declutter i915_regs.h.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8e76007a361bd3ca8dd8913281854886b3a1954c.1692287501.git.jani.nikula@intel.com