drm/i915/backlight: split out backlight registers to a separate file

Declutter i915_reg.h by splitting backlight registers to a separate
file. Also include the utility pin definitions, even though they are
used for non-backlight things too.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220815094838.3511723-1-jani.nikula@intel.com