drm/dp: add a number of DP 2.0 DPCD definitions

Prepare for future with DP 2.0 DPCD definitions, with a couple of
related drive-by cleanups. No functional changes.

v2: Send the version that actually builds.

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/20200918114017.30198-2-jani.nikula@intel.com