drm/dp: add subheadings to DPCD address definitions

Add the subheadings from the DP spec. No functional changes.

Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200918114017.30198-1-jani.nikula@intel.com