drm/dp: add DPCD definitions from eDP 1.4

Add a number of DPCD definitions from eDP 1.4.

v2: s/DP_ALPM_LOCK_TIMEOUT_ERROR_STATUS/DP_ALPM_LOCK_TIMEOUT_ERROR/
   (Sonika)

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