drm/dp_helper: add defines for DP 1.2 and MST support. (v2)

This just adds the defines from the DP 1.2 spec, which we
will use later.

fix some DP MST to 1.2 MST

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Todd Previte <tprevite@gmail.com>
Reviewed-by: Jingoo Han <jg1.han@samsung.com>