drm/i915/dp: Add register definitions for Intel HDR backlight interface

No functional changes yet, this just adds definitions for all of the
known DPCD registers used by Intel's HDR backlight interface. Since
we'll only ever use this in i915, we just define them in
intel_dp_aux_backlight.c

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: thaytan@noraisin.net
Cc: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201204223603.249878-7-lyude@redhat.com