drm/dp: add DPCD definitions from DP 1.1 and 1.2a

Add a number of DPCD definitions from DP 1.1 and 1.2a.

v2: drop wrong DP version reference, rename DP training set macros
(Sonika).

Reviewed-by: Sonika Jindal <sonika.jindal@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>